Tissue heating and ablation systems and methods using electrode structures with distally oriented porous regions

ABSTRACT

Systems and methods for heating or ablating body tissue use a porous electrode structure in which the porous section of the structure occupies more of the distal region of the structure than the proximal region. In a preferred embodiment, at least +E,fra 1/3+EE rd of the proximal region of the structure is free of pores. The porous section can be either ultraporous or microporous.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Applicationshaving Ser. Nos. 60/010,223, 60/010,225 and 60/010,354, all of whichwere filed on Jan. 19, 1996.

FIELD OF THE INVENTION

The invention generally relates to electrode structures deployed ininterior regions of the body. In a more specific sense, the inventionrelates to electrode structures deployable into the heart for diagnosisand treatment of cardiac conditions.

BACKGROUND OF THE INVENTION

The treatment of cardiac arrhythmias requires electrodes capable ofcreating tissue lesions having a diversity of different geometries andcharacteristics, depending upon the particular physiology of thearrhythmia to be treated.

For example, a conventional 8 F diameter/4 mm long cardiac ablationelectrode can transmit radio frequency energy to create lesions inmyocardial tissue with a depth of about 0.5 cm and a width of about 10mm, with a lesion volume of up to 0.2 cm³. These small and shallowlesions are desired in the sinus node for sinus node modifications, oralong the A-V groove for various accessory pathway ablations, or alongthe slow zone of the tricuspid isthmus for atrial flutter (AFL) or AVnode slow pathways ablations.

However, the elimination of ventricular tachycardia (VT) substrates isthought to require significantly larger and deeper lesions, with apenetration depth greater than 1.5 cm, a width of more than 2.0 cm, witha lesion volume of at least 1 cm³.

There also remains the need to create lesions having relatively largesurface areas with shallow depths.

One proposed solution to the creation of diverse lesion characteristicsis to use different forms of ablation energy. However, technologiessurrounding microwave, laser, ultrasound, and chemical ablation arelargely unproven for this purpose.

The use of active cooling in association with the transmission of DC orradio frequency ablation energy is known to force the electrode-tissueinterface to lower temperature values, As a result, the hottest tissuetemperature region is shifted deeper into the tissue, which, in turn,shifts the boundary of the tissue rendered nonviable by ablation deeperinto the tissue. An electrode that is actively cooled can be used totransmit more ablation energy into the tissue, compared to the sameelectrode that is not actively cooled. However, control of activecooling is required to keep maximum tissue temperatures safely belowabout 100° C., at which tissue desiccation or tissue boiling is known tooccur.

Another proposed solution to the creation of larger lesions, either insurface area and/or depth, is the use of substantially larger electrodesthan those commercially available. Yet, larger electrodes themselvespose problems of size and maneuverability, which weigh against a safeand easy introduction of large electrodes through a vein or artery intothe heart.

A need exists for multi-purpose cardiac ablation electrodes that canselectively create lesions of different geometries and characteristics.Multi-purpose electrodes would possess the requisite flexibility andmaneuverability permitting safe and easy introduction into the heart.Once deployed inside the heart, these electrodes would possess thecapability to emit energy sufficient to create, in a controlled fashion,either large and deep lesions, or small and shallow lesions, or largeand shallow lesions, depending upon the therapy required.

SUMMARY OF THE INVENTION

One aspect of the invention provides a porous electrode assembly havinga wall comprising a distal region, a proximal region, and an exteriorthat peripherally surrounds an interior area. The wall is adapted toselectively assume an expanded geometry having a first maximum diameterand a collapsed geometry having a second maximum diameter less than thefirst maximum diameter. A medium containing ions substantially fills theinterior area when the wall is in the expanded geometry. An elementelectrically couples the medium to a source of electrical energy. Thewall includes a porous section sized to pass ions contained in themedium, to thereby enable ionic transport of electrical energy from thesource through the medium and porous section to the exterior of thewall. According to this aspect of the invention, the porous sectionoccupies more of the distal region of the wall than the proximal region.

In a preferred embodiment, at least 1/3rd of the proximal region of thewall is free of pores.

In one arrangement, the porous section comprises at least first andsecond porous zones spaced apart by a third zone free of pores. In oneimplementation, the first and second porous zones are circumferentiallyspaced apart by a third zone about the axis of the electrode structure.In another implementation, the first and second porous zones are spacedapart by the third zone along the axis of the electrode structure.

In one embodiment, the porous section or, if more than one, the porouszones are ultraporous. In another embodiment, the porous section orzones are microporous.

Another aspect of the invention provides systems and methods forablating tissue using the porous electrode assembly as above described.In a preferred embodiment, the systems and methods specify a firstelectrical resistivity for the porous electrode assembly to achieve afirst tissue lesion characteristic. In this embodiment, the systems andmethods further specify a second electrical resistivity for the porouselectrode assembly different than the first electrical resistivity toachieve a second tissue lesion characteristic different than the firstlesion characteristic. By specifying electrical resistivities for theporous electrode assembly, the systems and methods selectively achieveeither a deep tissue lesion geometry or a shallow tissue lesiongeometry.

Other features and advantages of the inventions are set forth in thefollowing Description and Drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a system for ablating heart tissue, whichincludes an expandable porous electrode structure that embodies thefeatures of the invention;

FIG. 2 is an enlarged side elevation view, with portions broken away, ofa porous electrode structure usable in association with the system shownin FIG. 1, with the electrode structure shown in its expanded geometry;

FIG. 3 is an enlarged side elevation view of the porous electrodestructure shown in FIG. 2, with the electrode structure shown in itscollapsed geometry;

FIG. 4 is a further enlarged, somewhat diagrammatic side view, withportions broken away, of the porous electrode structure shown in FIG. 2;

FIG. 5 is an enlarged side elevation view, with portions broken away, ofa porous electrode structure usable in association with the system shownin FIG. 1, with the electrode structure shown in its expanded geometrydue to the presence of an interior spline support structure;

FIG. 6 is an enlarged side section view of the porous electrodestructure shown in FIG. 5, with the electrode structure shown in itscollapsed geometry due to the manipulation of an exterior slidingsheath;

FIG. 7 is an enlarged side elevation view, with portions broken away, ofa porous electrode structure usable in association with the system shownin FIG. 1, with the electrode structure shown in its expanded geometrydue to the presence of an interior interwoven mesh support structure;

FIG. 8 is an enlarged, somewhat diagrammatic enlarged view, takengenerally along line 8--8 of FIG. 4, showing the ionic current densitiesacross the pores of the electrode body shown in FIG. 4;

FIG. 9 is a graph showing the relationship between sensed impedance andionic transport through the pores of the electrode body shown in FIG. 4;

FIG. 10 is an enlarged side elevation view, with portions broken away,of an alternative porous electrode structure usable in association withthe system shown in FIG. 1, with the electrode structure comprising aporous foam body shown in its expanded geometry;

FIG. 11 is an enlarged side view of a porous electrode structure usablein association with the system shown in FIG. 1, with the pores of thestructure arranged in a bulls eye pattern on the distal end of the body;

FIG. 12 is an enlarged side view of a porous electrode structure usablein association with the system shown in FIG. 1, with the pores of thestructure arranged in circumferentially spaced segments along the sideof the body;

FIG. 13 is a side view, with portions broken away, showing the use ofmultiple chambers to convey liquid to the segmented pore regions shownin FIG. 12;

FIG. 14 is an enlarged side elevation view, with portions broken away,of a porous electrode structure usable in association with the systemshown in FIG. 1, which also carries nonporous electrode elements;

FIG. 15 is an enlarged side section view of a porous electrodestructure, which also carries electrode elements formed by wire snakedthrough the body of the structure;

FIG. 16 is an enlarged side elevation view, with portions broken away,of a porous electrode structure with interior pacing/sensing electrodes;

FIGS. 17 and 18 are diagrammatic representations of the tissuetemperature profiles associated with a porous electrode structure whenoperated under different conditions;

FIG. 19 is a somewhat diagrammatic view of a fixture and mandrel forforming a hemispherical geometry for the distal end of a porouselectrode body from a flat sheet of porous material;

FIG. 20 is a side sectional view of the fixture and mandrel shown inFIG. 19 in the process of forming the hemispherical distal end geometryin a flat sheet of porous material;

FIG. 21 is an enlarged side section view of the sheet of porous materialafter formation of the hemispherical distal end geometry;

FIG. 22 is a somewhat diagrammatic view of a finishing fixture forforming the hemispherical geometry for the proximal end of the porouselectrode body from the preformed sheet shown in FIG. 21;

FIG. 23 is an elevation view of the porous electrode body after havingbeen formed by the devices shown in FIGS. 19 to 22;

FIG. 24 is a somewhat diagrammatic view of an expandable finishingfixture that can be used instead of the finishing fixture shown in FIG.22 for forming the hemispherical geometry for the proximal end of theporous electrode body from the preformed sheet shown in FIG. 21;

FIG. 25 is an elevation view of the porous electrode body after havingbeen formed by the expandable fixture shown in FIG. 24;

FIG. 26 is a somewhat diagrammatic view of two preformed hemisphericalbody sections of porous electrode body before being joined together intoa composite porous electrode body;

FIG. 27 is a side elevation view of the composite porous electrode bodyformed by joining the two hemispherical sections shown in FIG. 26together along a circumferential seam;

FIG. 28A is a side section view showing the eversion of the porouselectrode body shown in FIG. 27 to place the circumferential seam on theinside of the body, away from direct tissue contact;

FIG. 28B is a side section view of the porous electrode body shown inFIG. 27 after having been everted to place the circumferential seam onthe inside of the body;

FIG. 29A is a side elevation view of a porous electrode body formed byjoining two hemispherical sections along an axial seam and aftereversion to place the axial seam on the inside of the body;

FIG. 29B is a top view of the porous electrode body with the evertedaxial seam shown in FIG. 29A;

FIG. 30 is a top view of a porous electrode body formed by joining twohemispherical sections along a main axial seam, with additionalintermediate axial seams to segment the body, after eversion to placethe axial seams on the inside of the body;

FIG. 31A is an enlarged side sectional view of a seam joining two sheetsof porous material together to form an electrode body, with atemperature sensing element encapsulated within the seam, and beforeeversion of the body;

FIG. 31B is a side elevation view of the seamed body, shown partially inFIG. 31A, with temperature sensing elements encapsulated in the seam,and before eversion of the body;

FIG. 31C is a side section view of the body shown in FIG. 31B aftereversion, placing the seam and the signal wires of the temperaturesensing elements inside the body;

FIG. 32A is a somewhat diagrammatic view of a porous electrode bodybeing formed from a regenerated cellulose material by dipping using anexpandable fixture;

FIG. 32B is the dip-formed body shown being formed in FIG. 32A, afterremoval of the expandable fixture and attachment of a fixture withsteering assembly to the distal end of the body, and before eversion;

FIG. 32C is the dip-formed body with distal fixture and steeringassembly, shown in FIG. 32B, after eversion;

FIG. 33 is an exemplary porous body formed in an elongated, cylindricalgeometry with changing radii along its length, forming the distal andproximal neck regions;

FIG. 34A is another exemplary porous body formed as a tube in anelongated, cylindrical geometry with constant radii along its length;and

FIG. 34B is the tube shown in FIG. 34A, with its distal end closed by aseam, and a port tube sealed to its proximal end for attachment to acatheter tube.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a tissue ablation system 10 that embodies the features ofthe invention.

The system 10 includes a flexible catheter tube 12 with a proximal end14 and a distal end 16. The proximal end 14 carries a handle 18. Thedistal end 16 carries an electrode structure 20, which embodies featuresof the invention. The purpose of the electrode structure 20 is totransmit ablation energy.

As FIGS. 2 and 3 best show, the electrode structure 20 includes anexpandable-collapsible body 22. The geometry of the body 22 can bealtered between a collapsed geometry (FIG. 3) and an enlarged, orexpanded, geometry (FIG. 2). In the illustrated and preferredembodiment, liquid pressure is used to inflate and maintain theexpandable-collapsible body 22 in the expanded geometry.

In this arrangement (see FIG. 2), the catheter tube 12 carries aninterior lumen 34 along its length. The distal end of the lumen 34 opensinto the hollow interior of the expandable-collapsible body 22. Theproximal end of the lumen 34 communicates with a port 36 (see FIG. 1) onthe handle 18. The liquid inflation medium (arrows 38 in FIG. 2) isconveyed under positive pressure through the port 36 and into the lumen34. The liquid medium 38 fills the interior of theexpandable-collapsible body 22. The liquid medium 38 exerts interiorpressure to urge the expandable-collapsible body 22 from its collapsedgeometry to the enlarged geometry.

This characteristic allows the expandable-collapsible body 22 to assumea collapsed, low profile (ideally, less than 8 French diameter, i.e.,less than about 0.267 cm) when introduced into the vasculature. Oncelocated in the desired position, the expandable-collapsible body 22 canbe urged into a significantly expanded geometry of, for example,approximately 7 to 20 mm.

As FIGS. 5 to 7 show, the structure 20 can include, if desired, anormally open, yet collapsible, interior support structure 54 to applyinternal force to augment or replace the force of liquid medium pressureto maintain the body 22 in the expanded geometry. The form of theinterior support structure 54 can vary. It can, for example, comprise anassemblage of flexible spline elements 24, as shown in FIG. 5, or aninterior porous, interwoven mesh or an open porous foam structure 26, asshown in FIG. 7.

In these arrangements (see FIG. 6), the internally supportedexpandable-collapsible body 22 is brought to a collapsed geometry, afterthe removal of the inflation medium, by outside compression applied byan outer sheath 28 (see FIG. 6), which slides along the catheter tube12. As FIG. 6 shows, forward movement of the sheath 28 advances it overthe expanded expandable-collapsible body 22. The expandable-collapsiblebody 22 collapses into its low profile geometry within the sheath 28.Rearward movement of the sheath 28 (see FIG. 5 or 7) retracts it awayfrom the expandable-collapsible body 22. Free from the confines of thesheath 48, the interior support structure 54 springs open to return theexpandable-collapsible body 22 to its expanded geometry to receive theliquid medium.

As FIG. 4 best shows, the structure 20 further includes an interiorelectrode 30 formed of an electrically conductive material carriedwithin the interior of the body 22. The material of the interiorelectrode 30 has both a relatively high electrical conductivity and arelatively high thermal conductivity. Materials possessing thesecharacteristics include gold, platinum, platinum/iridium, among others.Noble metals are preferred.

An insulated signal wire 32 is coupled to the electrode 30. The signalwire 32 extends from the electrode 30, through the catheter tube 12, toan external connector 38 on the handle 18 (see FIG. 1). The connector 38electrically couples the electrode 30 to a radio frequency generator 40.

In the preferred and illustrated embodiment (see FIG. 1), a controller42 is associated with the generator 40, either as an integrated unit oras a separate interface box. The controller 42 governs the delivery ofradio frequency ablation energy to the electrode 30 according topreestablished criteria. Further details of this aspect of the system 10will be described later.

According to the invention, the liquid medium 38 used to fill the body22 includes an electrically conductive liquid. The liquid 38 establishesan electrically conductive path, which conveys radio frequency energyfrom the electrode 30. In conjunction, the body 22 comprises anelectrically non-conductive thermoplastic or elastomeric material thatcontains pores 44 on at least a portion of its surface. The pores 44 ofthe porous body 22 (shown diagrammatically in enlarged form in FIG. 4for the purpose of illustration) establishes ionic transport of ablationenergy from the electrode 30, through the electrically conductive medium38, to tissue outside the body.

Preferably, the liquid 38 possesses a low resistivity to decrease ohmicloses, and thus ohmic heating effects, within the body 22. In theillustrated and preferred embodiment, the liquid 38 also serves theadditional function as the inflation medium for the body, at least inpart.

The composition of the electrically conductive liquid 38 can vary. Inthe illustrated and preferred embodiment, the liquid 38 comprises ahypertonic saline solution, having a sodium chloride concentration at ornear saturation, which is about 9% weight by volume. Hypertonic salinesolution has a low resistivity of only about 5 ohm·cm, compared to bloodresistivity of about 150 ohm·cm and myocardial tissue resistivity ofabout 500 ohm·cm.

Alternatively, the composition of the electrically conductive liquidmedium 38 can comprise a hypertonic potassium chloride solution. Thismedium, while promoting the desired ionic transfer, requires closermonitoring of rate at which ionic transport occurs through the pores 44,to prevent potassium overload. When hypertonic potassium chloridesolution is used, it is preferred keep the ionic transport rate belowabout 10 mEq/min.

The system 10 as just described is ideally suited for ablatingmyocardial tissue within the heart. In this environment, a physicianmoves the catheter tube 12 through a main vein or artery into a heartchamber, while the expandable-collapsible body 22 of the electrodestructure 20 is in its low profile geometry. Once inside the desiredheart chamber, the expandable-collapsible body 22 is enlarged into itsexpanded geometry and the region containing pores 44 is placed intocontact with the targeted region of endocardial tissue.

Due largely to mass concentration differentials across the pores 44,ions in the medium 38 will pass into the pores 44, because ofconcentration differential-driven diffusion. Ion diffusion through thepores 44 will continue as long as a concentration gradient is maintainedacross the body 22. The ions contained in the pores 44 provide the meansto conduct current across the body 22.

Radio frequency energy is conveyed from the generator 40 to theelectrode 30, as governed by the controller 42. When radio frequency(RF) voltage is applied to the electrode 30, electric current is carriedby the ions within the pores 44. The RF currents provided by the ionsresult in no net diffusion of ions, as would occur if a DC voltage wereapplied, although the ions do move slightly back and forth during the RFfrequency application. This ionic movement (and current flow) inresponse to the applied RF field does not require perfusion of liquid inthe medium 38 through the pores 44.

The ions convey radio frequency energy through the pores 44 into tissueto a return electrode, which is typically an external patch electrode(forming a unipolar arrangement). Alternatively, the transmitted energycan pass through tissue to an adjacent electrode in the heart chamber(forming a bipolar arrangement). The radio frequency energy heats thetissue, mostly ohmically, forming a lesion.

The electrical resistivity of the body 22 has a significant influence onthe lesion geometry and controllability. It has been discovered thatablation with devices that have a low-resistivity body 22 requires moreRF power and results in deeper lesions. On the other hand, devices thathave a high-resistivity body 22 generate more uniform heating,therefore, improve the controllability of the lesion. Because of theadditional heat generated by the increased body resistivity, less RFpower is required to reach similar tissue temperatures after the sameinterval of time. Consequently, lesions generated with high-resistivitybodies 22 usually have smaller depth.

Generally speaking, lower resistivities values for the body 22 belowabout 500 ohm·cm result in deeper lesion geometries. Likewise, higherresistivities for the body 22 at or above about 500 ohm·cm result inmore shallow lesion geometries.

The electrical resistivity of the body 22 can be controlled byspecifying the pore size of the material, the porosity of the material,and the water adsorption characteristics (hydrophilic versushydrophobic) of the material.

Specifying Pore Size

The size of the pores 44 in the body 22 can vary. Pore diameters smallerthan about 0.1 μm, typically used for blood oxygenation, dialysis, orultrafiltration, can be used for ionic transfer according to theinvention. These small pores, which can be visualized by high-energyelectron microscopes, retain macromolecules, but allow ionic transferthrough the pores in response to the applied RF field, as abovedescribed. With smaller pore diameters, pressure driven liquid perfusionthrough the pores 44 is less likely to accompany the ionic transport,unless relatively high pressure conditions develop within the body 22.

Larger pore diameters, typically used for blood microfiltration, canalso be used for ionic transfer according to the invention. These largerpores, which can be seen by light microscopy, retain blood cells, butpermit passage of ions in response to the applied RF field. Generallyspeaking, pore sizes below 8 μm will block most blood cells fromcrossing the membrane. With larger pore diameters, pressure drivenliquid perfusion, and the attendant transport of macromolecules throughthe pores 44, is also more likely to occur at normal inflation pressuresfor the body 22.

Still larger pore sizes can be used, capable of accommodating formedblood cell elements. However, considerations of overall porosity,perfusion rates, and lodgment of blood cells within the pores of thebody 22 must be taken more into account as pore size increase.

Conventional porous, biocompatible membrane materials used for bloodoxygenation, dialysis, blood filtration such as plasmapheresis can serveas the porous body 22. Such membrane materials can be made from, forexample, regenerated cellulose, nylon, polycarbonate, polyvinylidenefluoride (PTFE), polyethersulfone, modified acrylic copolymers, andcellulose acetate.

Alternatively, porous or microporous materials may be fabricated byweaving a material (such as nylon, polyester, polyethylene,polypropylene, fluorocarbon, fine diameter stainless steel, or otherfiber) into a mesh having the desired pore size and porosity. The use ofwoven materials is advantageous, because woven materials are veryflexible as small diameter fibers can be used to weave the mesh. Byusing woven materials, uniformity and consistency in pore size also canbe achieved.

Spectrum Medical Industries, Inc. (Houston, Tex.) commercially suppliesnylon and polyester woven materials with pore sizes as small as 5 μmwith porosities of 2%. Stainless steel woven materials with pore sizesas small as 30 μm with porosities of 30% can also be obtained fromSpectrum Medical Industries, Inc. Manufacturers, such as Tetko, alsoproduce woven materials meeting the desired specifications. Wovenmaterials with smaller pore sizes may be achieved depending on thematerial.

Woven meshes may be fabricated by conventional techniques, includingsquare mesh or twill mesh. Square mesh is formed by conventional "overand under" methods. Twill mesh is formed by sending two fibers over andunder. The materials may be woven into a 3-dimensional structure, suchas a tube or a sphere. Alternatively, the materials may be woven into aflat, 2-dimensional sheet and formed (heat forming, thermal bonding,mechanical deformation, ultrasonic welding etc.) into the desired3-dimensional geometry of the body 22.

Pore size can be specified using bubble point measurements. The bubblepoint value is defined as the pressure required to force liquid throughthe membrane, which is a function mainly of pore size (given the samewater adsorption characteristic). The standard for measuring bubblepoint value is ASTM F316-80.

Pore size correlates with the expected liquid flow resistance of themembrane. As a general proposition, larger pores allow more liquid toflow through the pores and at higher flow rates. Likewise, smaller poreslimit the volume and rate of liquid perfusion through the pores. At apoint, a pore will be small enough to effectively block liquidperfusion, except at very high pressure, while nevertheless enablingionic transport to occur in the manner described above.

Low or essentially no liquid perfusion through the pores 44 ispreferred. Limited or essentially no liquid perfusion through the pores44 is beneficial for several reasons. First, it limits salt or wateroverloading, caused by transport of the hypertonic solution into theblood pool. This is especially true, should the hypertonic solutioninclude potassium chloride, as observed above.

Furthermore, limited or essentially no liquid perfusion through thepores 44 allows ionic transport to occur without disruption. Whenundisturbed by attendant liquid perfusion, ionic transport creates acontinuous virtual electrode 48 (see FIG. 8) at the body 22-tissueinterface. The virtual electrode 48 efficiently transfers RF energywithout need for an electrically conductive metal surface.

The bubble point value in psi for a given porous material also aids inspecifying the nature of ionic transport the porous material supports,thereby indicating its suitability for tissue ablation.

When the bubble point value for a given porous material exceeds thepressure required to inflate the body 22 (i.e., inflation pressure), itis possible to pressure inflate the body 22 into its expanded geometry,without promoting pressure-driven liquid perfusion through the pores 44of the material. Specifying a material with a bubble point value greaterthan body inflation pressure assures that ionic transfer through thepores 44 occurs without attendant liquid perfusion through the pores 44.

A bubble point value that is significantly less than the body inflationpressure also indicates that the body 22 containing the porous materialmay never reach its intended expanded geometry, because of excessiveliquid perfusion through the pores 44.

On the other hand, the bubble point value of the porous material shouldnot exceed the tensile strength of the porous material. By specifyingthis relationship between bubble point value and tensile strength,liquid perfusion will occur before abnormally high pressures develop,lessening the chance that the body 22 will rupture.

The bubble point value specification mediates against the use of largerpore size materials. Larger pore size materials pose problems ofinflation and excessive fluid perfusion through the membrane.

Specifying Porosity

The placement of the pores 44 and the size of the pores 44 determine theporosity of the body 22. The porosity represents the space on the body22 that does not contain material, or is empty, or is composed of pores44. Expressed as a percentage, porosity represents the percent volume ofthe body 22 that is not occupied by the body material.

For materials having a porosity greater than about 10%, porosity P (in%) can be determined as follows: ##EQU1## where: ρ_(b) is the density ofthe body 22 as determined by its weight and volume, and

ρ_(m) is the density of the material from which the body 22 is made.

To derive porosity for materials having a porosity of less than about10%, a scanning electron microscope can be used to obtain the number ofpores and their average diameter. Porosity P (in %) is then derived asfollows: ##EQU2## where: N is the pore density and equals (P_(n) /a),

P_(n) is the number of pores in the body 22, a is the total porous areaof the body 22 (in cm²), and

π is the constant 3.1416 . . . ,

d is the average diameter of the pores (in cm).

The magnitude of the porosity affects the liquid flow resistance of thebody 22, as discussed above. The equivalent electrical resistivity ofthe body 22 also depends on its porosity. Low-porosity materials havehigh electrical resistivity, whereas high-porosity materials have lowelectrical resistivity. For example, a material with 3% porosity, whenexposed to 9% hypertonic solution (resistivity of 5 ohm·cm), may have anelectrical resistivity comparable to that of blood or tissue (between150 and 450 ohm·cm).

The distribution of pores 44 for a given porosity also affects theefficiency of ionic transport. Given a porosity value, an array ofnumerous smaller pores 44 is preferred, instead of an array of fewer butlarger pores. The presence of numerous small pores 44 distributescurrent density so that the current density at each pore 44 is less.With current density lessened, the ionic flow of electrical energy totissue occurs with minimal diminution due to resistive heat loss.

An array of numerous smaller pores 44 is also preferred, instead of anarray of fewer but larger pores, because it further helps to imposefavorably large liquid flow resistance. The presence of numerous smallpores 44 limits the rate at which liquid perfusion occurs through eachpore 44.

A dynamic change in resistance across a body 22 can be brought about bychanging the diameter of the body 22 made from a porous elasticmaterial, such as silicone. In this arrangement, the elastic body 22 ismade porous by drilling pores of the same size in the elastic materialwhen in a relaxed state, creating a given porosity. As the elastic body22 is inflated, its porosity remains essentially constant, but the wallthickness of the body 22 will decrease. Thus, with increasing diameterof the body 22, the resistance across the body 22 decreases, due todecreasing wall thickness and increasing surface area of the body 22. Asthe surface area of the body 22 increases by a factor of two, thethickness of the body 22 will decrease by a factor of two, resulting ina decrease in resistance by a factor of four.

As a result, the desired lesion geometry may be specified according tothe geometry of the body 22. This enables use of the same porous body 22to form small lesions, shallow and wide lesions, or wide and deeplesions, by controlling the geometry of the body 22.

Preferably, the porous body 22 should possess consistent pore size andporosity throughout the desired ablation region. Without consistent poresize and porosity, difference in electrical resistance of the body 22throughout the ablation region can cause localized regions of highercurrent density and, as a result, higher temperature. If the differencein electrical resistance is high enough, the lesion may not betherapeutic, because it may not extend to the desired depth or length.Furthermore, nonuniform areas of low porosity in the body 22 canthemselves experience physical damage as a result of the localizedheating effects.

Specifying Water Adsorption Characteristics

The porous material for the body 22 may be either hydrophobic orhydrophilic. However, the water adsorption characteristics of the porousmaterial also affect the electrical resistivity of the material.

For materials of the same pore size and porosity, materials that arehydrophilic possess greater capacity to provide ionic transfer ofradiofrequency energy without significant liquid flow through thematerial. Ions suspended in the medium are more likely to fully occupythe pores of a hydrophilic material in the absence of a driving pressureexceeding the bubble point value of the material, compared tohydrophobic materials. The presence of these ions within the pores inthe hydrophilic materials provides the capacity of ionic current flowwith no need for liquid perfusion through the pores. As a result, poresizes may be decreased more readily with hydrophilic materials, therebyraising the bubble point value to minimize liquid perfusion, withoutadversely affecting desired ionic current-carrying capacities.Furthermore, the relationship between porosity and resistivity is moredirect in the case of hydrophilic materials than with hydrophobicmaterials.

Some forms of nylon (e.g., nylon 6 or nylon 6/6) are examples ofhydrophilic materials having high water adsorption suitable for use as aporous electrode. The nylon sample identified in Example 3 below has4.0% to 4.2% moisture adsorption at 65% relative humidity and atemperature of 20° C.

Nevertheless, conventional medical grade "balloon" materials, such asPET and PeBax, are hydrophobic. Ions in the medium are less likely tooccupy the pore of a hydrophobic membrane, absent a driving pressureexceeding the bubble point value of the material, compared to ahydrophilic material. As a result, hydrophobic materials are more likelyto require liquid flow through the pores to carry ions into the pores,to thereby enable transmission of electrical energy across the porousmaterial. With such materials, the inflation pressure of the body shouldexceed the bubble point value to enable effective ionic transport.

Furthermore, due to the higher surface tension of hydrophobic material,which tends to restrict ion flow into the pores, hydrophobic materialsalso exhibit a greater tendency to cause material breakdown at thepores, compared with hydrophilic materials. The large potentialdifferences across each pore in a hydrophobic material may causedissociation of water molecules, dielectric breakdown of the membranematerial, and localized overheating. The breakdown is associated withhigh temperature effects and, depending on the material, can open up thepores, burn the material surrounding the pores, and generally degradethe material. In addition, material breakdown can produce hazardoustissue effects similar to DC ablation, such as tissue charring.

Therefore, changing the water adsorption characteristics of a porousmaterial from more hydrophobic to more hydrophilic can offset undesiredelectrical characteristics, without changing pore size or porosity. Forexample, the incidence of material breakdown due to high currentdensities and potential drops at the pores can be reduced by increasingthe porosity of the material. However, the incidence of materialbreakdown can be reduced or eliminated without altering the porosity, byselecting a material that is hydrophilic; for example, materials such asregenerated cellulose, nylon 6, and nylon 6/6, which typically have highwater adsorption. Alternatively, coatings or surface treatments may beapplied to a less hydrophilic material making it more hydrophilic. Forexample, some materials can be dipped into a specially formulatedhydrophilic coating and exposed to ultraviolet light to bind the coatingto the material surface. This approach is especially advantageous whenconventional "balloon" materials are used for the body 22, provided thecoating withstands ablation temperatures without degrading.

Other measures can be employed to offset other undesired electricalcharacteristics due to pore size or porosity or water adsorptionproperties. For example, for larger pore materials, or when poroushydrophilic materials are used, the perfusion rate can be controlled bycontrolling fluid pressures across the body 22.

Alternatively, for larger pore materials, or when porous hydrophilicmaterials are used, a material can be added to the hypertonic solutionto increase its viscosity, thereby decreasing its perfusion rate.Examples of materials that can be added to increase viscosity includeionic contrast (radiopaque) substances or nonionic glycerol orconcentrated mannitol solutions.

For example, the electrical performance of woven materials having largerpore sizes may be aided by the addition of an ionic radiopaque contrastmaterial like Renografin® -76. By adding a radiopaque material to theaqueous solution, the body 22 may be seen under fluoroscopy (orechocardiography, depending on the contrast material). The flowresistance of the porous material will effectively increase, due to theincreased viscosity of the medium.

The use of ionic materials to increase viscosity need not excessivelyincrease the resistivity of the membrane, depending on the concentrationof the ionic material. The following Table 1 summarizes the results ofin vitro experiments, using an ionic radiopaque material with a wovennylon 13.0 mm disk probe.

                  TABLE 1                                                         ______________________________________                                        Effects of Ion Contrast Material on Ablation                                  with a Woven Nylon Disk                                                       Fluid    Set       Average Average                                                                              Lesion                                                                              Lesion                                Medium   Temperature                                                                             Power   Impedance                                                                            Depth Length                                ______________________________________                                        9% NaCl  90° C.                                                                           23 W     68 Ω                                                                          9.9 mm                                                                              21.0 mm                               50%-9%NaCl                                                                             90° C.                                                                           13 W     85 Ω                                                                          8.4 mm                                                                              16.6 mm                               50%-Contrast                                                                  Contrast 90° C.                                                                           14 W    120 Ω                                                                          8.6 mm                                                                              17.9 mm                               Material                                                                      ______________________________________                                         NOTE: All lesion dimensions are based on the 60° C. discoloration      characteristic.                                                          

Table 1 shows that the ionic contrast medium can reduce the powerrequired to achieve equivalent ablation results and still create desiredlesions.

For porous materials, either hydrophilic or hydrophobic, the system 10can include a device to sense impedance proximate to the body-tissueinterface. As FIG. 9 shows, impedance decreases with increasing liquidperfusion flow rate, until a limit point is reached, at which impedancestabilizes despite increasing perfusion rates. By sensing impedance, itis possible to control perfusate flow between a minimum flow rateR_(MIN) (at which impedance is too high) and a maximum flow rate R_(MAX)(above which potential salt or water overload conditions come intoexistence).

The surface area of the electrode 30 bathed in the electricallyconductive medium within the body can be increased to enhance ionictransfer. However, the desired characteristics of small geometrycollaspsibility and overall flexibility of the body impose practicalconstraints upon electrode size.

The proximity of the electrode 30 to the pores 44 of the body 22 alsoenhances the efficiency of ionic transfer through the electricallyconductive medium. Again, the structural characteristics of presenting aflexible, small collapsed profile create practical constraints upon thisconsideration.

Forming the Body 22

The expandable-collapsible body 22 can be formed about the exterior orinterior of a glass mold. In this arrangement, the external dimensionsof the mold match the desired expanded geometry of theexpandable-collapsible body 22. The mold is dipped in a desired sequenceinto a solution of the body material until the desired wall thickness isachieved. The mold is then etched away, leaving the formedexpandable-collapsible body 22.

Alternatively, the expandable-collapsible body 22 may also be blowmolded from extruded tubing. In this arrangement, the body 22 is sealedat one end using adhesive or thermal fusion. The opposite end of thebody 22 is left open. The sealed expandable-collapsible body 22 isplaced inside the mold. An inflation medium, such as high pressure gasor liquid, is introduced through the open tube end. The mold is exposedto heat as the tube body 22 is inflated to assume the mold geometry. Theformed expandable-collapsible body 22 is then pulled from the mold.

The porosity of the body 22 can be imparted either before or aftermolding by CO₂ laser, eximer laser, YAG laser, high power YAG laser,electronic particle bombardment, and the like.

As earlier discussed, coatings or surface treatments may also be appliedto make the surface more hydrophilic to improve the electricalproperties of the body 22 for tissue ablation.

Commercially available porous materials can also be formed into the body22. For those materials having poor bonding properties that are formedby chemical processes, such as the regenerated cellulose, the materialmay be chemically formed into a three-dimensional geometry by a dippingprocess (as generally shown in FIG. 32A and as will be described later),injection molding, or by varying the diameter and geometry duringextrusion.

For those materials that can be thermally bonded, laser welded,ultrasonically welded, and adhesively bonded, there are various waysthat make use of these bonding or welding techniques to form athree-dimensional geometry from a sheet of the material may be employed.Fixtures and mandrels can be used to form the body 22 in conjunctionwith heat and pressure.

FIGS. 19 to 23 show a preferred way for forming a sheet of porousmaterial 200 into the desired three dimensional geometry of an ablationbody 22. As FIG. 19 shows, the sheet 200 is placed over a forming cavity202 on a fixture 204. The geometry of the forming cavity 202 correspondsto the geometry desired for the distal end of the body 22. In theillustrated embodiment, the geometry is generally hemispherical.

As FIG. 20 shows, a forming mandrel 206 presses a section 208 of thesheet 200 into the forming cavity 202. The geometry of the formingmandrel 206 matches the hemispherical geometry of the forming cavity202. The mandrel 206 nests within the cavity 202, sandwiching thematerial section 208 between them. This sets by pressure the desiredhemispherical shape to the material section 208. Either the formingmandrel 206 or the forming cavity 202, or both, may be heated to providean additional thermal set to the material section 208 within the cavity202. Pressure and, optionally, heat within the cavity 202 shape thematerial section 208 from a planar geometry into the desiredhemispherical geometry (see FIG. 21).

The sheet with the preformed section 208 is removed from the fixture 204and mounted upon a finishing fixture 210 (see FIG. 22). The finishingfixture 210 includes a distal end 212 having a geometry that matches thegeometry of the preformed section 208. The section 208 fits on thedistal fixture end 212.

The finishing fixture 210 includes a proximal end 214 that has thegeometry desired for the proximal end of the body 22, which in theillustrated embodiment is hemispherical, too. The sheet 200 is snuglydraped about the proximal end 214 of the fixture 210.

The finishing fixture 210 includes a base region 216, about which theremaining material of the sheet 200 is gathered in overlapping pleats218. The sheet 200 thereby tightly conforms to the entire geometry ofthe fixture 210.

The finishing fixture 210 may be heated to aid in providing anadditional thermal set to the sheet 200 in the desired geometry of thebody 22. A clam shell mold (not shown) may also be fastened about thefixture 210 to facilitate the shaping process.

The sheet material, now shaped as the porous body 22 (see FIG. 23) isslipped from the fixture 210. The material pleats 218 that had beengathered about the base region 216 of the fixture 210 are bondedtogether, for example, by thermal bonding or ultrasonically welding.This forms a reduced diameter neck region 220 in the body 22 tofacilitate attachment of the body 22 to the distal end of a cathetertube.

Before pleating, the sheet ends 217 may be cut into sections to minimizethe amount of material which accumulates during the pleating process.After pleating, the excess material may be back-folded and bonded to theneck region 220 and/or otherwise trimmed to form a smooth transitionbetween the neck region 220 and the distal region 208.

Alternatively, after removal from the fixture 204, the sheet 200 withpreformed section 208, can be wrapped about an expandable fixture 222(see FIG. 24). The proximal ends 217 of the sheet 200 are snugly tiedabout the neck of the fixture 222 by a tie member 223.

The fixture 222 comprises a balloon (made, for example, from a Teflonmaterial) or the like, which can be expanded using gas or liquid intothe geometry desired for the body 22. The sheet 200 is thereby shaped bythe expanding fixture 222 to take the desired geometry.

Before or during expansion of the fixture 222, heat may be applied tothe ends 217 of the sheet 200 to soften the material to aid the shapingprocess. External pressure may also be applied to the proximal ends ofthe sheet 200 to aid in creating the neck region 220 having the desiredreduced diameter. This also helps to prevent "bunching" of material atthe proximal ends 217.

The fixture 222 itself may also be heated by using heated gas or liquidto expand the fixture. The heat provides an additional thermal set tothe sheet 200 in the desired geometry of the body 22. An external clamshell mold (not shown) may also be fastened about the fixture 222 tofacilitate the shaping process. Alternatively. an external shell of amaterial such as glass, which may be etched away, may be used to impartthe desired final geometry.

Throughout any heating process used in forming the body 22 using eitherfixture 204 or 222, a heat sink (not shown) may be used to cool thepreformed distal section 208 so that the pore sizes do not changesignificantly during a heating process.

Alternatively, the heating effects on the pore size may be estimated andaccounted for in forming the sheet 200 in the first instance, beforeshaping into the body 22. For example, if the pores open during shaping,the pores may be formed during manufacture proportionally smaller, totake into account the increase in size during shaping. Thus, the desiredpore size is ultimately achieved while shaping the sheet into the body22.

After the shaping process, the fixture 222 is deflated and withdrawn(see FIG. 25). The formed body 22 remains.

In yet another alternative process (see FIGS. 26 and 27), the body 22can be formed by joining two preformed sections 225 along acircumferential seam 224. In the illustrated embodiment, the sections225 are formed as hemispheres in the manner shown in FIGS. 19 to 21,with excess material about the periphery of the section 225 cut away.The sections 225 could likewise be preformed by a molding process,depending upon the properties of the material.

The seam 224 joining the two sections 225 is formed through thermalbonding, ultrasonically welding, laser welding, adhesive bonding,sewing, or the like, depending upon the properties of the material. Thebonding or sewing method employed is selected to assure that the seamforms an air-tight and liquid-sealed region. The tensile strength of theseam 224 should also exceed the bubble point value of the porousmaterial.

Alternatively, two generally circular planar sections of porousmaterial, cut to size from a sheet, can be joined about theirperipheries by a seam, without prior shaping. This creates a normallycollapsed disk enclosing an open interior. The introduction of air orliquid into the open interior during use causes the disk to expand intothe geometry desired for the body 22. The disk could also enclose aninterior support structure 54 (as generally shown in FIGS. 5 to 7),which shapes the disk to the desired geometry.

Preferably, after joining the hemispherical or planar sections 225 atthe seam 224, excess material extending beyond to the seam 224 is cutaway. Still, as contact between tissue and the somewhat roughenedsurface region of the seam 224 could cause trauma, the joined sections225 are preferably everted (see FIG. 28A). Eversion locates the seam 224within the interior of the body 22 (as FIG. 28B shows), away from directcontact with tissue.

As FIG. 28A shows, the joined sections 225 can be everted by creating asmall hole 250 at one end 252, inserting a pull-wire 254 and attachingit to the opposite end 256, then pulling the opposite end 256 throughthe hole 250. This turns the attached hemispherical sections 225 insideout.

In the foregoing embodiments, the circumferential seam 224 extends aboutthe axis of the body 22. Alternatively (as FIGS. 29A and 29B show),seams 226 can extend along the axis of the body 22 to join two or moresections 228, either planar or preformed into a three dimensional shape,into the body 22. Mating fixtures (not shown) can be used, each carryinga body section 228, to hold the sections 228 stationary while heat orultrasonic energy is applied to create the seam 226.

As FIG. 30 shows, other axially extending seams 230 may also be placedwithin a sheet of porous material, not to join the sheet to anothersheet, but rather to segment the sheet. Further details about segmentedporous electrodes will be discussed later. For the purpose ofillustration, FIG. 30 somewhat exaggerates the hemispherical protrusionof the segments along the seams 226 and 230.

Preferably, the resulting body 22 is everted, as just described, toplace the axially extending seams 226 or 230 inside the body 22.

It should be appreciated that the sections 225 or 228 shown in FIGS. 26to 30, whether planar or preforming in three dimensional geometries,need not be made of the same material. Materials of different porouscharacteristics can be joined by seams in the manner just described.Alternatively, porous materials may be joined by seams to nonporousmaterials, which can themselves be either electrically conductive orelectrically insulating. Or, still alternatively, electricallyconductive materials can be joined by seams to insulating materials, toprovide double sided electrode bodies, one (electrically conductive) forcontacting tissue, and the other (electrically insulating) for exposureto the blood pool. Virtually any flexible material suitable for use inassociation with an electrode body can be combined using seams accordingto this aspect of the invention. Also, it should be realized that thenumber of sections joined together by seams to form a compositeelectrode body can vary.

Various specific geometries, of course, can be selected, as well. Thepreferred geometry is essentially spherical and symmetric, with a distalspherical contour, as FIG. 2 shows. However, nonsymmetric ornonspherical geometries can be used. For example, theexpandable-collapsible body 22 may be formed with a flattened distalcontour, which gradually curves or necks inwardly for attachment withthe catheter tube 12. Elongated, cylindrical geometries can also beused, such as shown in FIGS. 33 and 34B, which will be discussed later.

FIG. 10 shows an alternative expandable-collapsible porous body 50. Inthis embodiment, the body 50 comprises open cell foam molded to normallyassume the shape of the expanded geometry. The electrode 30 isencapsulated within the foam body 50. The hypertonic liquid medium 38 isintroduced into the foam body 50, filling the open cells, to enable thedesired ionic transport of ablation energy, as already described. Thetransport of ions using the foam body 50 will also occur if the body 50includes an outer porous skin 51(as the right side of FIG. 10 shows),which can provide a porosity less than the porosity of the foam body 50to control the perfusion rate.

In this arrangement, a sliding sheath (as previously described) can beadvanced along the catheter tube 12 to compress the foam body 50 intothe collapsed geometry. Likewise, retraction of the sheath removes thecompression force. The foam body 50, free of the sheath, springs open toreturn the expandable-collapsible body 50 back to the expanded geometry.

In the illustrated and preferred embodiment, a distal steering mechanism52 (see FIG. 1) enhances the manipulation of the porous electrodestructure 20 or 50, both during and after deployment. The steeringmechanism 52 can vary. In the illustrated embodiment (see FIG. 1), thesteering mechanism 52 includes a rotating cam wheel 56 coupled to anexternal steering lever 58 carried by the handle 18. The cam wheel 56holds the proximal ends of right and left steering wires 60. The wires60 pass with the ablation energy signal wires 32 through the cathetertube 12 and connect to the left and right sides of a resilient bendablewire or leaf spring (not shown) adjacent the distal tube end 16. Furtherdetails of this and other types of steering mechanisms are shown inLundquist and Thompson U.S. Pat. No. 5,254,088, which is incorporatedinto this Specification by reference.

As shown in FIG. 1, the leaf spring of the steering mechanism 52 iscarried within in the distal end 16 of the catheter tube 12. As FIG. 1shows, forward movement of the steering lever 58 pulls on one steeringwire 60 to flex or curve the leaf spring, and, with it, the distalcatheter end 16 and the electrode structure 20, in one direction.Rearward movement of the steering lever 58 pulls on the other steeringwire 60 to flex or curve the leaf spring 62, and, with it, the distalcatheter end 16 and the electrode structure 20, in the oppositedirection.

Alternatively, as FIG. 32C shows, a steerable leaf spring 268 is part ofa distal fixture 270, which is itself attached to the distal end of theporous body 22. In this arrangement, the leaf spring 268 extends beyondthe distal catheter end 16 within a tube 272 inside the porous body 22.The steering wires 60 and 62 attached to the leaf spring 268 also passthrough the tube 272. The proximal end of the leaf spring 268 is securedto a hub 274 attached to the distal catheter end 16.

In this arrangement, forward and rearward movement of the steering lever58 on the handle 18 bends the leaf spring 268 in opposite directionswithin the body 22. The leaf spring 268 moves the distal fixture 270 anddeforms the porous body 22 in the direction that the leaf spring 268bends.

In either arrangement, the steering mechanism 54 is usable whether theexpandable-collapsible body is in its collapsed geometry or in itsexpanded geometry.

FIGS. 32A and 32B show a preferred way of securing the distal fixture270 and leaf spring 268 to a porous body 22. In FIG. 32A, the porousbody 22 is formed by dipping an expandable fixture 276 having a desiredgeometry into solution of regenerated cellulose 278. The details of suchan expandable fixture 276 have already been described in another contextand are shown in FIGS. 24 and 25. It should be appreciated that theporous body 22 can be formed in various other ways, as alreadydescribed.

As FIG. 32B shows, the fixture forms a dip-formed porous body 22 havinga proximal neck region 280 and a distal neck region 282. After moldingthe body 22, the expandable fixture 276 is collapsed and withdrawn, asFIG. 32B also shows.

As FIG. 32B shows, the distal neck region 282 is secured about thedistal fixture 270, for example using adhesive or a sleeve 288 that issecured by adhesive bonding, thermal bonding, mechanical bonding,screws, winding, or a combination of any of these.

The distal fixture 270 has, preattached to it, the leaf spring 268 andassociated components, already described. When initially secured to thefixture 270, the proximal neck region 280 of the body 22 is oriented ina direction opposite to the leaf spring 268.

After securing the distal neck region 282 to the fixture 270, the body22 is everted about the distal fixture 270 over the leaf spring 268, asFIG. 32C shows. The proximal end of the leaf spring 268 is secured tothe hub 274 carried by the distal catheter end 16. The everted proximalneck region 280 is then secured to the distal catheter end by use of asleeve 286. The sleeve 286 can be secured about the catheter tube invarious ways, including adhesive bonding, thermal bonding, mechanicalbonding, screws, winding, or a combination of any of these.

Various alternative ways of attaching a porous electrode body to thedistal end of a catheter are disclosed in copending patent applicationentitled "Stem Elements for Securing Tubing and Electrical Wires toExpandable-Collapsible Electrode Structures," (application Ser. No.08/630,113).

As will be described in greater detail later, the distal fixture 270 canalso serve as a nonporous electrically conductive region on the porousbody 22. Similar fixtures 270 can be located elsewhere on the porousbody 22 for the same purpose.

A stilette (not shown) may also be attached to the distal fixture 270instead of or in combination with the leaf spring 268. From there, thestilette extends inside the body 22 (following eversion), through thecatheter tube 12, to a suitable push-pull controller on the handle 18(not shown). The stilette is movable along the axis of the catheter tube12 to push and pull axially upon the distal fixture 270, therebyelongating or shortening the body 22.

Further details concerning the attachment of a distal fixture to anelectrode body are shown in copending patent application entitled"Expandable-Collapsible Electrode Structures With Distal End Steering orManipulation," (application Ser. No. 08/628,980).

FIGS. 33 and 34A/34B show exemplary electrode bodies having elongated,cylindrical geometries, which can be associated with various distalfixtures in the manner shown and attached to distal catheter ends 16 inthe manner shown in FIGS. 32B and 32C.

In FIG. 33, the body 290 is formed by extrusion, dipping, or moldinginto an elongated geometry with varying radii to form the distal andproximal neck regions 280 and 282. A suitable distal fixture 270 (shownin phantom lines) can be secured within the distal neck region 282 andthe elongated body 22 everted to complete the assembly, in the mannershown in FIGS. 32B and 32C. The proximal neck region 280 can then besecured to a distal catheter end 16 in the manner shown in FIG. 32C.

In FIGS. 34A and 34B, the body 22 is formed from a tube 292 of materialformed by extrusion, molding, or dipping with a uniform radius (shown inFIG. 34A). In this arrangement (see FIG. 34B), a seam 294, formed themanner previously disclosed, closes the distal end of the tube 292. Theproximal end of the tube 292 is sealed about a tubular port 296, forattachment to the distal catheter end 16. Alternatively, the distal endof the tube 292 can be sealed about a distal fixture 270 (shown inphantom lines in FIG. 34B). In the latter case, the tube 292 is evertedabout the distal fixture 270 before attachment to the catheter distalend 16.

The pattern of pores 44 that define the porous region of the body mayvary. Preferably, as generally shown in FIGS. 2 and 3, the region of atleast the proximal 1/3rd surface of the expandable-collapsible body 22is free of pores 44.

The absence of pores 44 on the at least proximal 1/3rd surface of theexpandable-collapsible body 22 is desirable for several reasons. Thisregion is not normally in contact with tissue, so the presence of thevirtual electrode boundary serves no purpose. Furthermore, this regionalso presents the smallest diameter. If electrically conductive, thisregion would possess the greatest current density, which is notdesirable. Keeping the proximal region of smallest diameter, which isusually free of tissue contact, free of pores 44 assures that themaximum current density will be distributed at or near the distal regionof the expandable-collapsible body 22, which will be in tissue contact.

When it is expected that ablation will occur with the distal region ofbody 22 oriented in end-on contact with tissue, the porous regionshould, or course, be oriented about the distal tip of theexpandable-collapsible body 22. For this end-on orientation, the porousregion may comprise a continuous cap deposited upon the distal 1/3rd to1/2 of the body 22, as FIGS. 2 and 3 show. However, when distal contactwith tissue is contemplated, the preferred embodiment (see FIG. 11)segments the electrically conductive porous region into separate energytransmission zones 62 arranged in a concentric "bulls eye" pattern aboutthe distal tip of the body 22.

When it is expected that ablation will occur with the side region of thebody 22 oriented in contact with tissue, the porous region is preferablysegmented into axially elongated energy transmission zones 62 (see FIG.12), which are circumferentially spaced about the distal 1/3rd to 1/2 ofthe body.

When the porous region comprises segmented zones 62 on the body 22, aninterior group of sealed bladders 64 (see FIG. 13) can be used toindividually convey liquid 38 to each porous region segment 62. Eachbladder 64 individually communicates with a lumen 66 to receive theelectrically conductive liquid for the one porous region 62 it services.The multiple lumens pass through the catheter tube 12. The multiplebladders 64 also provide the ability to more particularly control thegeometry of the expanded body 22, by selectively inflating with theliquid some but not all the bladders 64.

The bladders 64 may be separately formed and inserted into the body 22,or they may be integrally formed during molding the mainexpandable-collapsible body 22.

As FIG. 12 shows, segmented porous zones 62 are also well suited for usein association with folding expandable-collapsible bodies 22. In thisarrangement, the regions that are free of pores comprise creased orfolding regions 68. To create these regions 68, the mold for the body 22has a preformed surface geometry such that the expandable-collapsiblematerial would be formed slightly thinner, indented, or ribbed along thedesired regions 68. The expandable-collapsible body 22 collapses aboutthese creased regions 68, causing the body 22 to circumferentially foldupon itself in a consistent, uniform fashion. The resulting collapsedgeometry can thus be made more uniform and compact.

It should be appreciated that the foldable body 22 shown in FIG. 12 canalso be used for other patterns of porous regions. The creased regions68 can also be provided with pores, if desired.

FIG. 14 shows an embodiment of an expandable-collapsible electrodestructure 70 that serves dual functions. The structure 70 includes anexpandable-collapsible body 22, as previously described, containing theinterior electrode 30. The body 22 contains an electrically conductivefluid 38, and also includes one or more porous regions 62 to enableionic transport of electrical energy, also as just described.

The structure 70 shown in FIG. 14 also includes one or more nonporous,electrically conductive regions 72 on the surface of the body 22. In oneembodiment (as FIG. 14 shows), the nonporous conductive regions 72comprise metal, such as gold, platinum, platinum/iridium, among others,deposited upon the expandable-collapsible body 22 by sputtering, vapordeposition, ion beam deposition, electroplating over a deposited seedlayer, or a combination of these processes. Alternatively, the nonporousconductive regions 72 can comprise thin foil affixed to the surface ofthe body. Still alternatively, the nonporous conductive regions cancomprise solid fixtures (like the distal fixture 270 shown in FIG. 32C)carried by the porous body 22 at or more locations. Signal wires (notshown) within the body are electrically coupled to the nonporousregions. The signal wires traverse the catheter tube 12 for coupling tothe connectors 38 carried by the handle 18.

In the preferred embodiment (see FIG. 15), the nonporous conductiveregions 72 comprise insulated signal wires 26 passed into the interiorof the body and then snaked through the body 22 at the desired point ofelectrical connection. The electrical insulation of the distal end ofthe snaked-through wire 26 is removed to exposed the electricalconductor, which is also preferably flattened, to serve as theconductive region 72. The flattened region 72 is affixed by anelectrically conductive adhesive 73 to body 22. Adhesive 73 is alsopreferable applied in the region of the body 22 where the wire 26 passesto seal it. The same signal wire 26 can be snaked through the body 22multiple times to establish multiple regions 72, if desired.

Various ways for attaching nonporous electrodes 72 and associated signalwires to an expandable-collapsible electrode body 22 are described incopending Patent Application entitled "Enhanced Electrical Connectionsfor Electrode Structures" (application Ser. No. 08/629,363).

The nonporous regions 72 can be used to sense electrical activity inmyocardial tissue. The sensed electrical activity is conveyed to anexternal controller, which processes the potentials for analysis by thephysician. The processing can create a map of electrical potentials ordepolarization events for the purpose of locating potential arrhythmiafoci. Once located with the nonporous regions 72, the porous regions 62can be used to convey radio frequency energy as previously described toablate the foci.

Alternatively, or in combination with sensing electrical activities, thenonporous regions 72 can be used to convey pacing signals. In this way,the nonporous regions can carry out pace mapping or entrainment mapping.

Preferably (see FIGS. 16), the interior surface of the body 22 carrieselectrodes 100 suitable for unipolar or bipolar sensing or pacing.Although these electrodes 100 are located on the interior surface of thebody 22, their ability for sensing or pacing is not impaired because ofthe good electrical conductive properties of the body 22.

Different electrode placements can be used for unipolar or bipolarsensing or pacing. For example, pairs of 2-mm length and 1-mm widthelectrodes 100 can be deposited on the interior surface of the body 22.Connection wires 102 can be attached to these electrodes 100. To preventthe hypertonic solution from electrically short-circuiting theseelectrodes, they have to be covered with an electrically insulatingmaterial 104 (e.g. epoxy, adhesive etc.). Preferably the interelectrodedistance is about 1 mm to insure good quality bipolar electrograms.Preferred placements of these interior electrodes are at the distal tipand center of the structure 22. Also, when multiple zones are used, itis desired to have the electrodes 100 placed in between the ablationregions.

It is also preferred to deposit opaque markers 106 on the interiorsurface of the body 22 so that the physician can guide the device underfluoroscopy to the targeted site. Any high-atomic weight material issuitable for this purpose. For example, platinum, platinum-iridium. canbe used to build the markers 106. Preferred placements of these markers106 are at the distal tip and center of the structure 22.

The expandable-collapsible structure 70 shown in FIG. 14 therebycombines the use of "solid" nonporous electrodes 72 with "liquid" orporous electrodes 62. The expandable-collapsible structure makespossible the mapping of myocardial tissue for therapeutic purposes usingone electrode function, and the ablation of myocardial tissue fortherapeutic purposes using a different electrode function.

In an alternative embodiment, the nonporous regions 72 of the structure70 can be used in tandem with the porous regions 62 to convey radiofrequency energy to ablate tissue. In this arrangement, the signal wiresserving the region 72 are electrically coupled to the generator 40 toconvey radio frequency energy for transmission by one or more regions72. At the same time, the interior electrode 30 receives radio frequencyenergy for transmission by the medium 38 through the porous body. Theionic transport across the porous structure surrounding the regions 72extends the effective surface area of the ablation electrode.

In this embodiment, the expandable-collapsible structure 70 shown inFIG. 14 thereby combines the use of electrodes 72 having a firsteffective surface area for sensing and mapping. The first effectivesurface area can be selectively increased for ablation purposes by ionictransport of a hypertonic liquid across a porous structure surroundingthe electrodes 72.

If liquid perfusion occurs through the pores, an interior electrode 30is not required to increase the effective electrode surface area of theregions. The liquid perfusion of the ionic medium through the pores atthe time the regions transmit radio frequency energy is itselfsufficient to increase the effective transmission surface area of theregions 72. However, if ionic transfer occurs without substantial liquidperfusion, it is believed that it would be advantageous in increasingthe effective surface area to also transmit radio frequency energy usingan interior electrode 30 at the same time that radio frequency is beingdelivered to the exterior regions 72 for transmission.

It should also be appreciated that, in this embodiment, the regions 72can themselves be made from a porous, electrically conducting material.In this way, ionic transport can occur across the regions 72 themselves.

As before described (see FIG. 1), a controller 32 preferably governs theconveyance of radio frequency ablation energy from the generator 30 tothe electrode carried within the body 22. In the preferred embodiment(see FIG. 2), the porous electrode structure 20 carries one or moretemperature sensing elements 104, which are coupled to the controller32.

The temperature sensing elements 104 can take the form of thermistors,thermocouples, or the equivalent. The sensing elements 104 are inthermal conductive contact with the exterior of the electrode structure20 to sense conditions in tissue outside the structure 20 duringablation.

Temperatures sensed by the temperature sensing elements 104 areprocessed by the controller 32. Based upon temperature input, thecontroller adjusts the time and power level of radio frequency energytransmissions by the electrode 30, to achieve the desired lesionpatterns and other ablation objectives.

Various ways for attaching temperature sensing elements to anexpandable-collapsible electrode body are described in copending PatentApplication entitled "Enhanced Electrical Connections for ElectrodeStructures" (application Ser. No. 08/639,363).

As FIGS. 31A, 31B, and 31C show, temperature sensing elements 104 canalso be positioned proximal to or within a seam 258 joining sheets 260and 262 of porous material together into a body 22. The formation ofsuch seams 258 has been already described and is also shown in FIGS. 26to 30.

As shown in FIGS. 31A and 31B, each temperature sensing element 104 isplaced on one sheet 260, and then covered by the other sheet 262. Thetwo sheets 260 and 262 are then seamed together, forming the body 22.The seam 258 encapsulates the sensing elements. The signal wire 264 foreach sensing element 104 extends free of the seam 258 to the exterior ofthe sheets 260 and 262, as FIGS. 31A and 31B show.

As previously described, the body 22 is preferably everted (see FIG.28A). As FIG. 31C shows, eversion locates both the seam 258 and theencapsulated signal wires 264 within the interior of the body 22. Thesignal wires 264 are passed through the neck 266 for coupling to thecontroller 32.

Instead of or in addition to the temperature sensing elements 104,pacing electrodes or sensing electrodes may be encapsulated within theseams 258 of everted electrode bodies 22 in the manner shown in FIGS.31A to 31C. In such arrangements, it is preferred to locate theelectrodes so that, after eversion, they are located within the seamsclose to the surface of the material where intended contact between thebody material and tissue is to take place.

Further details of the use of multiple ablation energy transmitterscontrolled using multiple temperature sensing elements are disclosed incopending U.S. patent application Ser. No. 08/286,930, filed Aug. 8,1994, and entitled "Systems and Methods for Controlling Tissue AblationUsing Multiple Temperature Sensing Elements".

EXAMPLE 1 In Vitro Analysis

Three electrode configurations were analyzed:

(1) a porous expandable-collapsible electrode structure made accordingto the invention, having a 13 mm disk-shaped body constructed fromdialysis tubing made from regenerated cellulose(manufactured bySpectra), with a molecular weight cut off of 12,000-14,000 Daltons, andusing 9% saline solution as the internal liquid medium;

(2) a sputtered platinum disk-shaped electrode body having a diameter of13 mm; and

(3) an expandable-collapsible hemispherical electrode structureconstructed of aluminum foil with a diameter of 10 mm.

A thermistor was embedded 0.5 mm into animal tissue (sheep) at theregion where maximum temperature conditions existed. For electrodes (1)and (2), the maximum temperature was at edges of the disk-shaped body,due to edge heating effects. For the electrode (3), the maximumtemperature was at the distal tip of the hemispherical body, wherecurrent densities are greatest.

Radio frequency electromagnetic power was regulated to maintain thethermistor temperature at 60° C., 70° C., 80° C., or 90° C. All lesiondimensions were measured based on the 60° C. isotherms, marking thediscoloration of tissue.

The following Table 2 lists the observed in vitro results.

                  TABLE 2                                                         ______________________________________                                                       Averege Average                                                                              Average                                                                             Lesion Lesion                                    Time    Power   Impedance                                                                            Temp  Depth  Length                             Electrode                                                                            (Sec)   (Watts) (Ohms) (°C.)                                                                        (mm)   (mm)                               ______________________________________                                        1      120     11      75     59    4.2    15.8                               1      120     21      72     68    6.4 ™                                                                             17.5                               1      120     17      69     78    8.0    17.1                               1      120     15      62     78    9.7 ™                                                                             18.3                               1      120     25      69     87    8.9 ™                                                                             19.5                               1      120     26      66     87    8.8 ™                                                                             20.8                               1      120     22      69     88    10.0 ™                                                                            19.1                               2      128      9      76     78    6.3    14.2                               3      120     23      69     78    6.0    13.7                               3      120     37      55     86    7.4    18.5                               ______________________________________                                         Note: ™ indicates that the lesion was transmural, so depths were           actually larger than measured.                                           

The porous electrode structure (1) was minimally affected by convectivecooling compared to normal metal ablation electrode, such as electrode(2). This was observed by varying fluid flow about the electrode duringa particular lesion and observing that, with a porous electrodestructure, no change in power was required to maintain thermistortemperature.

Table 2 shows that the porous electrode structure created lesions atleast as large as a metal coated electrode structures when regulatingpower based on tissue temperature. The porous electrode structure alsohad reasonable impedance levels compared to the metal coated electrodestructures.

This Example demonstrates that a porous electrode structure can createlesions deeper than 1.0 cm in a controlled fashion to ablate epicardial,endocardial, or intramural VT substrates.

The dialysis tubing forming the porous electrode structure has a highwater adsorption characteristic. The dialysis tubing becomessignificantly more flexible when exposed to water. The molecular weightcutoff was 12,000 to 14,000 daltons. Larger or smaller molecular weightcut-offs are available from Spectrum. The conversion from molecularweight cutoff to estimated pore size for the dialysis tubing tested is100,000 daltons equals 0.01 μm; 50,000 daltons equals 0.004 μm; 10,000daltons equals 0.0025 μm; 5,000 daltons equals 0.0015 μm, as taken fromSpectrum Medical Instruments brochure entitled"Dialysis/Ultrafiltration," 94/95 (p 10).

The dialysis tubing possesses a hydrophilic nature and high porositydespite low pore sizes. As a result, the bubble point value is extremelyhigh and the resistivity is low enough to not require fluid flow duringdelivery of radiofrequency energy.

EXAMPLE 2 Finite Element Analysis

A three-dimensional finite element model was created for a porouselectrode structure having a body with an elongated shape, with a totallength of 28.4 mm, a diameter of 6.4 mm, and a body wall thickness of0.1 mm. A 0.2-mm diameter metal wire extended within the length of thebody to serve as an interior electrode connected to an RF energy source.The body was filled with 9% hypertonic solution, having an electricalresistivity of 5.0 ohm·cm. The porous body of the structure was modeledas an electric conductor. Firm contact with cardiac tissue was assumedalong the entire length of the electrode body lying in a plane beneaththe electrode. Contact with blood was assumed along the entire length ofthe electrode body lying in a plane above the electrode. The blood andtissue regions had resistivities of 150 and 500 ohm·cm, respectively.

Analyses were made based upon resistivities of 1.2 k-ohm·cm and 12k-ohm·cm for the electrode body.

Table 3 shows the depth of the maximum tissue temperature when RFablation power is applied to the porous electrode at various powerlevels and at various levels of resistivity for the porous body of theelectrode.

                  TABLE 3                                                         ______________________________________                                        Resistivity of                       Depth of                                 the Povous                   Maximum Maximum                                  Body       Power   Time      Tissue  Tissue                                   (k-ohm · cm)                                                                    (Watts) (Sec)     Temp (°C.)                                                                     Temp(cm)                                 ______________________________________                                        1.2        58      120       96.9    1.1                                      1.2        58      240       97.9    1.4                                      12         40      120       94.4    0.8                                      12         40      240       95.0    1.0                                      ______________________________________                                    

FIG. 17 shows the temperature profiles when power is applied to theelectrode at 58 watts for 240 seconds when the porous body has aresistivity of 1.2 k-ohm·cm. The depth of 50° C. isotherm in FIG. 17 is1.4 cm.

FIG. 18 shows the temperature profiles when power is applied to theelectrode at 40 watts for 240 seconds when the porous body has aresistivity of 12 k-ohm·cm. Depth of 50° C. isotherm in FIG. 18 is 1.0cm.

In all cases, the maximal temperature is located at the interfacebetween tissue and the opposite end edges of the elongated porousstructure. This dictates that the preferred location for temperaturesensing elements for an elongated geometry of the porous body is at eachend edge of the body. Preferably, each edge should carry at least onetemperature sensing element, and multiple sensing elements should belocated in diametrically opposite sides to assure that at least one ofthem faces tissue.

The data also show that the hottest region is not moved deep into thetissue, as would be observed with metal surface electrodes. The hottestregion consistently resides at the tissue-electrode body interface fordirect sensing. This feature reduces the difference between sensedtemperature and actual hottest tissue temperature potentially to atheoretical 0° C., although somewhat higher differentials may beencountered given other aspects of the instrumentation.

The porous electrode body with higher resistivity body (see FIG. 18)generated more uniform temperature profiles, compared to a porous bodyhaving the lower resistivity value (see FIG. 17). Due to additionalheating generated at the tissue-electrode body interface with increasedelectrode body resistivity, less power was required to reach samemaximal temperature. The consequence was that the lesion depthdecreased.

As before explained, by selecting the resistivity of the body 22, thephysician can significantly influence lesion geometry. The use of alow-resistivity body 22 results in deeper lesions, and vice versa. Thefollowing Table 4, based upon empirical data, demonstrates therelationship between body resistivity and lesion depths.

                  TABLE 4                                                         ______________________________________                                        Resistivity                                                                           Power    Temperature                                                                             Lesion Depth                                       (ohm · cm)                                                                   (Watts)  (°C.)                                                                            (cm)       Time (sec)                              ______________________________________                                          850   94       97        1.2        120                                       1200  58       97        1.1        120                                     12,000  40       95        0.8        120                                     ______________________________________                                    

Because of the reduced thermal conductivity of the porous electrodestructure, when compared to nonporous, metallic surface electrodes,lesion formation is expected to be less sensitive to dynamic blood flowconditions around the electrode. The application of ablation energythrough porous electrodes can be more closely controlled to obtaindesired lesion characteristics, particularly when shallow atrial lesionare desired.

The following Table 5, based upon empirical data, demonstrates thereduced sensitivity of porous electrode structures to convective coolingconditions due to changes in blood flow rates.

                  TABLE 5                                                         ______________________________________                                        Convective Cooling                                                                       Maximum     Power                                                  Conditions Temperature (°C.)                                                                  (Watts)  Lesion Depth (cm)                             ______________________________________                                        Normal     97          94       1.2                                           50% Reduced                                                                              98          94       1.3                                           75% Reduced                                                                              95          79       0.8                                           ______________________________________                                    

The use of porous electrode structures provides structural benefits. Itisolates possible adherence problems that may be associated with theplacement of metal, electrically conductive shells to the outside ofexpandable-collapsible bodies. Porous electrode structures also avoidpotential problems that tissue sticking to exterior conductive materialscould create.

In addition to these structural benefits, the temperature control of theablation process is improved. When using a conventional metal electrodeto ablate tissue, the tissue-electrode interface is convectively cooledby surrounding blood flow. Due to these convective cooling effects, theregion of maximum tissue temperature is located deeper in the tissue. Asa result, the temperature conditions sensed by sensing elementsassociated with metal electrode elements do not directly reflect actualmaximum tissue temperature. In this situation, maximum tissuetemperature conditions must be inferred or predicted from actual sensedtemperatures. Using a porous electrode structure 20 or 70, convectivecooling of the tissue-electrode interface by the surrounding blood flowis minimized. As a result, the region of maximum temperature is locatedat the interface between tissue and the porous electrode. As a result,the temperature conditions sensed by sensing elements associated withporous electrode elements will more closely reflect actual maximumtissue.

EXAMPLE 3

In Vitro experiments were performed to compare hydrophilic materials(Hphl) versus hydrophobic materials (HPhb) in terms of their use asporous tissue ablation elements. Table 6 summarizes the results.

                                      TABLE 6                                     __________________________________________________________________________    Summary of Porous Ablation Materials                                                               Bubble                                                                   Pore point                                                                             No Flow                                                                            Impedance                                                                          Mat'l                                                                             Lesion                                 Mat'l                                                                              Mfgr HPhb                                                                             HPhl                                                                             Size value                                                                             Impedance                                                                          w/Flow                                                                             Brkdwn                                                                            Depth                                  __________________________________________________________________________    Dialysis                                                                           Spectrum                                                                              .check mark.                                                                     .025                                                                             μm                                                                           High                                                                              87 Ω                                                                         87 Ω                                                                         No  13.8 mm                                Tubing                                                                        Nylon                                                                              Spectrum                                                                              .check mark.                                                                     5  μm                                                                           Med 68 Ω                                                                         68 Ω                                                                         No   9.9 mm                                Mesh                                                                          Stain-St                                                                           Spectrum   30 μm                                                                           Low 67 Ω                                                                         67 Ω                                                                         No   9.7 mm                                Mesh                                                                          Polycarb                                                                           Millipore                                                                             .check mark.                                                                     1.2                                                                              μm                                                                           High 14                                                                           78 Ω                                                                         78 Ω                                                                         No  11.6 mm                                Film                 psi                                                      Polyvin-                                                                           Millipore                                                                          .check mark.                                                                        5  μm                                                                           High                                                                              >300                                                                             Ω                                                                         84 Ω                                                                         Yes 10.7 mm                                ylidene                                w/ flow                                Fluoride                                                                      PTFE Millipore  5  μm                                                                           High                                                                              >300                                                                             Ω                                                                         >300                                                                             Ω                                                                         N/A NONE                                   Polyethers                                                                         Gelman  .check mark.                                                                     5  μm                                                                           Med 1-                                                                            80 Ω                                                                         80 Ω                                                                         No  10.6 mm                                ulfone               6 psi                                                    Polyethers                                                                         Gelman  .check mark.                                                                     0.1                                                                              μm                                                                           High                                                                              >300                                                                             Ω                                                                         >300                                                                             Ω                                                                         N/A NONE                                   ulfone                                                                        Modified                                                                           Gelman                                                                             .check mark.                                                                        10 μm                                                                           Med 68 Ω                                                                         68 Ω                                                                         Yes  3.9 mm                                Acrylic              1-6 psi                                                  copolymer                                                                     Modified                                                                           Gelman                                                                             .check mark.                                                                        5  μm                                                                           High                                                                              >300                                                                             Ω                                                                         70 Ω                                                                         Yes 11.0 mm                                Acrylic                                w/ flow                                copolymer                                                                     Modified                                                                           Gelman                                                                             .check mark.                                                                        10 μm                                                                           High                                                                              >300                                                                             Ω                                                                         61 Ω                                                                         Yes 11.3 mm                                Acrylic w/                             w/ flow                                backing                                                                       PTFE Pore Tech                                                                          .check mark.                                                                        1  μm                                                                           High                                                                              >300                                                                             Ω                                                                         >300                                                                             Ω                                                                         N/A NONE                                   Cellulose                                                                          Goodfello                                                                             .check mark.                                                                     Very High                                                                              >300                                                                             Ω                                                                         >300                                                                             Ω                                                                         N/A NONE                                   Acetate                                                                            w          low                                                           __________________________________________________________________________     Note: "Mat'l Brkdwn" refers to the presence of material breakdown, as         described above.                                                         

Table 6 demonstrates that pore sizes may be decreased using hydrophilicmaterials, thereby minimizing or stopping liquid perfusion through theporous material, while still enabling ionic transport through themembrane.

Hydrophobic porous materials make possible the realization of highresistivity porous electrodes. On the other hand, hydrophilic porousmaterials make possible the realization of low resistivity porouselectrodes.

Obtaining Desired Lesion Characteristics

As the foregoing tables demonstrate, the same expandable-collapsibleporous electrode structure 20 is able to selectively form lesions thatare either wide and shallow or large and deep. Various methodologies canbe used to control the application of radio frequency energy to achievethis result.

A. D_(50C) Function

In one representative embodiment, the controller 42 includes an input300 (see FIG. 1) for receiving from the physician a desired therapeuticresult in terms of (i) the extent to which the desired lesion shouldextend beneath the tissue-electrode interface to a boundary depthbetween viable and nonviable tissue and/or (ii) a maximum tissuetemperature developed within the lesion between the tissue-electrodeinterface and the boundary depth.

The controller 42 also includes a processing element 302 (see FIG. 1),which retains a function that correlates an observed relationship amonglesion boundary depth, ablation power level, ablation time, actualsub-surface tissue temperature, and electrode temperature. Theprocessing element 302 compares the desired therapeutic result to thefunction and selects an operating condition based upon the comparison toachieve the desired therapeutic result without exceeding a prescribedactual or predicted sub-surface tissue temperature.

The operating condition selected by the processing element 302 cancontrol various aspects of the ablation procedure, such as controllingthe ablation power level, limiting the ablation time to a selectedtargeted ablation time, limiting the ablation power level subject to aprescribed maximum ablation power level, and/or the orientation of theporous region 44 of the body 22, including prescribing a desiredpercentage contact between the region 44 and tissue. The processingelement 302 can rely upon temperature sensors carried by or otherwiseassociated with the expandable-collapsible structure 20 that penetratethe tissue to sense actual maximum tissue temperature. Alternatively,the processing element 302 can predict maximum tissue temperature basedupon operating conditions.

In the preferred embodiment, the electrode structure 20 carries at leastone temperature sensing element 104 to sense instantaneous localizedtemperatures (T1) of the thermal mass of the region 44. The temperatureT1 at any given time is a function of the power supplied to theelectrode 30 by the generator 40.

The characteristic of a lesion can be expressed in terms of the depthbelow the tissue surface of the 50° C. isothermal region, which will becalled D_(50C). The depth D_(50C) is a function of the physicalcharacteristics of the porous region 44 (that is, its electrical andthermal conductivities, resistivities, and size); the percentage ofcontact between the tissue and the porous region 44; the localizedtemperature T1 of the thermal mass of the region 44; the magnitude of RFpower (P) transmitted by the interior electrode 30, and the time (t) thetissue is exposed to the RF power.

For a desired lesion depth D_(50C), additional considerations of safetyconstrain the selection of an optimal operating condition among theoperating conditions listed in the matrix. The principal safetyconstraints are the maximum tissue temperature TMAX and maximum powerlevel PMAX.

The maximum temperature condition TMAX lies within a range oftemperatures which are high enough to provide deep and wide lesions(typically between about 85° C. and 95° C.), but which are safely belowabout 100° C., at which tissue desiccation or tissue micro-explosionsare known to occur. It is recognized that TMAX will occur a distancebelow the electrode-tissue interface between the interface and D_(50C).

The maximum power level PMAX takes into account the physicalcharacteristics of the interior electrode 30 and the power generationcapacity of the RF generator 40.

These relationships can be observed empirically and/or by computermodeling under controlled real and simulated conditions, as theforegoing examples illustrate. The D_(50C) function for a given porousregion 44 can be expressed in terms of a matrix listing all or some ofthe foregoing values and their relationship derived from empirical dataand/or computer modeling.

The processing element 302 includes in memory this matrix of operatingconditions defining the D_(50C) temperature boundary function, asdescribed above for t=120 seconds and TMAX=95° C. and for an array ofother operating conditions.

The physician also uses the input 300 to identify the characteristics ofthe structure 20, using a prescribed identification code; set a desiredmaximum RF power level PMAX; a desired time t; and a desired maximumtissue temperature TMAX.

Based upon these inputs, the processing element 302 compares the desiredtherapeutic result to the function defined in the matrix. The generator42 selects an operating condition to achieve the desired therapeuticresult without exceeding the prescribed TMAX by controlling the functionvariables.

This arrangement thereby permits the physician, in effect, to"dial-a-lesion" by specifying a desired D_(50C).

Further details of deriving the D_(50C) function and its use inobtaining a desired lesion pattern are found in copending U.S.application Ser. No. 08/431,790, filed May 1, 1995, entitled "Systemsand Methods for Obtaining Desired Lesion Characteristics While AblatingBody Tissue," which is incorporated herein by reference.

B. Segmented Regions: Duty Cycle Control

Various RF energy control schemes can also be used in conjunction withsegmented porous patterns shown in FIG. 11 (the axially spaced,bull's-eye pattern of zones) and FIG. 12 (the circumferentially spacedzones) For the purpose of discussion, the porous zones 44 (which willalso be called electrode regions) will be symbolically designated E(J),where J represents a given zone 44 (J=1 to N).

As before described, each electrode region E(J) has at least onetemperature sensing element 104, which will be designated S(J,K), whereJ represents the zone and K represents the number of temperature sensingelements on each zone (K=1 to M).

In this mode, the generator 40 is conditioned through an appropriatedpower switch interface to deliver RF power in multiple pulses of dutycycle 1/N.

With pulsed power delivery, the amount of power (P_(E)(J)) conveyed toeach individual electrode region E(J) is expressed as follows:

    P.sub.E(J) αAMP.sub.E(J).sup.2 ×DUTYCYCLE.sub.E(J)

where:

AMP_(E)(J) is the amplitude of the RF voltage conveyed to the electroderegion E(J), and

DUTYCYCLE_(E)(J) is the duty cycle of the pulse, expressed as follows:##EQU3## where: TON_(E)(J) is the time that the electrode region E(J)emits energy during each pulse period,

TOFF_(E)(J) is the time that the electrode region E(J) does not emitenergy during each pulse period.

The expression TON_(E)(J) +TOFF_(E)(J) represents the period of thepulse for each electrode region E(J).

In this mode, the generator 40 can collectively establish duty cycle(DUTYCYCLE_(E)(J)) of 1/N for each electrode region (N being equal tothe number of electrode regions).

The generator 40 may sequence successive power pulses to adjacentelectrode regions so that the end of the duty cycle for the precedingpulse overlaps slightly with the beginning of the duty cycle for thenext pulse. This overlap in pulse duty cycles assures that the generator40 applies power continuously, with no periods of interruption caused byopen circuits during pulse switching between successive electroderegions.

In this mode, the temperature controller 42 makes individual adjustmentsto the amplitude of the RF voltage for each electrode region(AMP_(E)(J)), thereby individually changing the power P_(E)(J) ofablating energy conveyed during the duty cycle to each electrode region,as controlled by the generator 40.

In this mode, the generator 40 cycles in successive data acquisitionsample periods. During each sample period, the generator 40 selectsindividual sensors S(J,K), and temperature codes TEMP(J) (highest ofS(J,K)) sensed by the sensing elements 104, as outputted by thecontroller 42.

When there is more than one sensing element 104 associated with a givenelectrode region (for example, when edge-located sensing elements areused, the controller 42 registers all sensed temperatures for the givenelectrode region and selects among these the highest sensed temperature,which constitutes TEMP(J).

In this mode, the generator 40 compares the temperature TEMP(J) locallysensed at each electrode E(J) during each data acquisition period to aset point temperature TEMP_(SET) established by the physician. Basedupon this comparison, the generator 40 varies the amplitude AMP_(E)(J)of the RF voltage delivered to the electrode region E(J), whilemaintaining the DUTYCYCLE_(E)(J) for that electrode region and all otherelectrode regions, to establish and maintain TEMP(J) at the set pointtemperature TEMP_(SET).

The set point temperature TEMP_(SET) can vary according to the judgmentof the physician and empirical data. A representative set pointtemperature for cardiac ablation is believed to lie in the range of 40°C. to 95° C., with 70° C. being a representative preferred value.

The manner in which the generator 40 governs AMP_(E)(J) can incorporateproportional control methods, proportional integral derivative (PID)control methods, or fuzzy logic control methods.

For example, using proportional control methods, if the temperaturesensed by the first sensing element TEMP(1)>TEMP_(SET), the controlsignal generated by the generator 30 individually reduces the amplitudeAMP_(E)(1) of the RF voltage applied to the first electrode region E(1),while keeping the duty cycle DUTYCYCLE_(E)(1) for the first electroderegion E(1) the same. If the temperature sensed by the second sensingelement TEMP(2)<TEMP_(SET), the control signal of the generator 30increases the amplitude AMP_(E)(2) of the pulse applied to the secondelectrode region E(2), while keeping the duty cycle DUTYCYCLE_(E)(2) forthe second electrode region E(2) the same as DUTYCYCLE_(E)(1), and soon. If the temperature sensed by a given sensing element is at the setpoint temperature TEMP_(SET), no change in RF voltage amplitude is madefor the associated electrode region.

The generator 40 continuously processes voltage difference inputs duringsuccessive data acquisition periods to individually adjust AMP_(E)(J) ateach electrode region E(J), while keeping the collective duty cycle thesame for all electrode regions E(J). In this way, the mode maintains adesired uniformity of temperature along the length of the ablatingelement.

Using a proportional integral differential (PID) control technique, thegenerator takes into account not only instantaneous changes that occurin a given sample period, but also changes that have occurred inprevious sample periods and the rate at which these changes are varyingover time. Thus, using a PID control technique, the generator willrespond differently to a given proportionally large instantaneousdifference between TEMP (J) and TEMP_(SET), depending upon whether thedifference is getting larger or smaller, compared to previousinstantaneous differences, and whether the rate at which the differenceis changing since previous sample periods is increasing or decreasing.

Further details of individual amplitude/collective duty cycle controlfor segmented electrode regions based upon temperature sensing are foundin copending U.S. application Ser. No. 08/439,824, filed May 12, 1995and entitled "systems and Methods for Controlling Tissue Ablation UsingMultiple Temperature Sensing Elements," which is incorporated herein byreference.

C. Segmented Regions: Differential Temperature Disabling

In this control mode, the controller 42 selects at the end of each dataacquisition phase the sensed temperature that is the greatest for thatphase (TEMP_(SMAX)). The controller 42 also selects for that phase thesensed temperature that is the lowest (TEMP_(SMIN)).

The generator compares the selected hottest sensed temperatureTEMP_(SMAX) to a selected high set point temperature TEMP_(HISET). Thecomparison generates a control signal that collectively adjusts theamplitude of the RF voltage for all electrode regions usingproportional, PID, or fuzzy logic control techniques.

In a proportion control implementation scheme:

(i) If TEMP_(SMAX) >TEMP_(HISET), the control signal collectivelydecreases the amplitude of the RF voltage delivered to all regions;

(ii) If TEMP_(SMAX) <TEMP_(HISET), the control signal collectivelyincreases the amplitude of the RF voltage delivered to all regions:

(iii) If TEMP_(SMAX) =TEMP_(HISET), no change in the amplitude of the RFvoltage delivered to all regions.

It should be appreciated that the generator can select for amplitudecontrol purposes any one of the sensed temperatures TEMP_(SMAX),TEMP_(SMIN), or temperatures in between, and compare this temperaturecondition to a preselected temperature condition.

The generator governs the delivery of power to the regions based upondifference between a given local temperature TEMP (J) and TEMP_(SMIN).This implementation computes the difference between local sensedtemperature TEMP(J) and TEMP_(SMIN) and compares this difference to aselected set point temperature difference ΔTEMP_(SET). The comparisongenerates a control signal that governs the delivery of power to theelectrode regions.

If the local sensed temperature TEMP(J) for a given electrode regionE(J) exceeds the lowest sensed temperature TEMP_(SMIN) by as much as ormore than ΔTEMP_(SET) (that is, if TEMP(J)-TEMP_(SMIN) ≧ΔTEMP_(SET)),the generator turns the given region E(J) off. The generator turns thegiven region E(J) back on when TEMP(J)-TEMP_(SMIN) <ΔTEMP_(SET).

Alternatively, instead of comparing TEMP(J) and TEMP_(SMIN), thegenerator can compare TEMP_(SMAX) and TEMP_(SMIN). When the differencebetween TEMP_(SMAX) and TEMP_(SMIN) equals or exceeds a predeterminedamount ΔTEMP_(SET), the generator turns all regions off, except theregion where TEMP_(SMIN) exists. The generator 30 turns these regionsback on when the temperature difference between TEMP_(SMAX) andTEMP_(SMIN) is less than ΔTEMP_(SET).

Further details of the use of differential temperature disabling arefound in copending U.S. patent application Ser. No. 08/286,930, filedAug. 8, 1994, and entitled "Systems and Methods for Controlling TissueAblation Using Multiple Temperature Sensing Elements," which isincorporated herein by reference.

D. PID Control

With porous electrode structures, the minimal effects of convectivecooling by the blood pool enables the use of actual sensed temperatureconditions as maximum tissue temperature TMAX, instead of predictedtemperatures. Because of this, such structures also lend themselves tothe use of a proportional integral differential (PID) control technique.An illustrative PID control techniques usable in association with theseelectrode structures are disclosed in copending U.S. patent applicationSer. No. 08/266,023, filed Jun. 27, 1994, entitled "Tissue Heating andAblation Systems and Methods Using Time-variable Set Point TemperatureCurves for Monitoring and Control."

Finally, it should be appreciated that interior electrodes 30 locatedwithin porous expandable-collapsible bodies can be used for mappingmyocardial tissue within the heart. In this use, the interior electrodessense electrical activity in the heart, which can take the form, forexample, of electrical potentials or tissue resistivity. The sensedelectrical activity is conveyed to an external controller, whichprocesses the sensed activities for analysis by the physician.

It should further be appreciated that interior electrodes 30 locatedwithin porous expandable-collapsible bodies can be used alternatively,or in combination with sensing electrical activities, to convey pacingsignals. In this way, the interior electrodes 30 can carry out pacemapping or entrainment mapping.

Various features of the invention are set forth in the following claims.

We claim:
 1. A porous electrode assembly for ablating body tissuecomprising a structure having a wall comprising a distal region and aproximal region, the wall having an exterior peripherally surrounding aninterior area, the wall adapted to selectively assume an expandedgeometry having a first maximum diameter and a collapsed geometry havinga second maximum diameter less than the first maximum diameter,a mediumcontaining ions substantially filling the interior area when the wall isin the expanded geometry, and an element for electrically coupling themedium to a source of electrical energy, wherein the wall includes aporous section sized to pass ions contained in the medium to therebyenable ionic transport of electrical energy sufficient to ablate tissuefrom the source through the medium and porous section to the exterior ofthe wall, the porous section occupying more of the distal region of thewall than the proximal region.
 2. A porous electrode assembly accordingto claim 1wherein at least 1/3rd of the proximal region of the wall isfree of pores.
 3. A porous electrode assembly according to claim1wherein the porous section comprises at least first and second porouszones spaced apart by a third zone free of pores.
 4. A porous electrodeassembly according to claim 3wherein the structure includes an axis, andwherein the first and second porous zones are circumferentially spacedapart by the third zone about the axis.
 5. A porous electrode assemblyaccording to claim 3wherein the structure includes an axis, and whereinthe first and second porous zones are spaced apart by the third zonealong the axis.
 6. A porous electrode assembly according to claim1wherein the wall is electrically conductive.
 7. A porous electrodeassembly according to claim 1and further including a radiopaque elementcarried by the structure.
 8. A porous electrode assembly according toclaim 1wherein the medium carries a radiopaque contrast substance.
 9. Aporous electrode assembly according to claim 1wherein the porous sectionhas an electrical resistivity of at least about 500 ohm·cm.
 10. A porouselectrode assembly according to claim 1wherein the porous section has anelectrical resistivity of less than about 500 ohm·cm.
 11. A porouselectrode assembly according to claim 1wherein the medium comprises ahypertonic solution.
 12. A porous electrode assembly according to claim11wherein the hypertonic solution includes sodium chloride.
 13. A porouselectrode assembly according to claim 12wherein the sodium chloride ispresent in a concentration at or near saturation.
 14. A porous electrodeassembly according to claim 12wherein the sodium chloride is present ina concentration of up to about 9% weight by volume.
 15. A porouselectrode assembly according to claim 11wherein the hypertonic solutionincludes potassium chloride.
 16. A porous electrode assembly accordingto claim 1wherein the element comprises an electrically conductiveelectrode carried within the interior area adapted to transmitelectrical energy.
 17. A porous electrode assembly according to claim16wherein the electrically conductive electrode comprises a nobel metal.18. A porous electrode assembly according to claim 16wherein theelectrically conductive electrode includes a material selected from thegroup consisting essentially of gold, platinum, platinum/iridium, andcombinations thereof.
 19. A porous electrode assembly according to claim1wherein the porous section is hydrophilic.
 20. A porous electrodeassembly according to claim 19wherein the medium occupies the interiorarea subject to interior pressure, wherein the porous section has abubble point value, and wherein the bubble point value exceeds theinterior pressure.
 21. A porous electrode assembly according to claim1wherein the porous section is hydrophobic.
 22. A porous electrodeassembly according to claim 21wherein the medium occupies the interiorarea subject to interior pressure, wherein the porous section has abubble point value, and wherein the bubble point value is equal to orless than the interior pressure.
 23. A porous electrode assemblyaccording to claim 1wherein the medium occupies the interior areasubject to interior pressure, wherein the porous section has a bubblepoint value, and wherein the bubble point value exceeds the interiorpressure.
 24. A porous electrode assembly according to claim 1whereinthe medium occupies the interior area subject to interior pressure,wherein the porous section has a bubble point value, and wherein thebubble point value is equal to or less than the interior pressure.
 25. Aporous electrode assembly according to claim 1wherein the porous sectionincludes a hydrophilic coating.
 26. A porous electrode assemblyaccording to claim 1wherein the wall is hydrophobic, and wherein theporous section includes a hydrophilic coating.
 27. A porous electrodeassembly according to claim 1wherein the porous section comprises anultraporous material.
 28. A porous electrode assembly according to claim1wherein the porous section comprises a microporous material.
 29. Asystem for heating body tissue comprisinga catheter tube having a distalend, a return electrode, a fluid source of a medium containing ions, aporous electrode on the distal end of the catheter tube adapted to beelectrically coupled to the return electrode through tissue, the porouselectrode comprising a wall comprising a distal region and a proximalregion, the wall having an exterior peripherally surrounding an interiorarea, the wall adapted to selectively assume an expanded geometry havinga first maximum diameter and a collapsed geometry having a secondmaximum diameter less than the first maximum diameter, and a lumencommunicating with the interior area and the fluid source to convey intothe interior area the medium containing ions, the wall including aporous section sized to pass ions contained in the medium, the poroussection occupying more of the distal region of the wall than theproximal region, and means for coupling the medium within the interiorarea to a source of electrical energy to establish ionic transport ofelectrical energy from the electrically conductive element through themedium to the exterior of the wall for transmission to the returnelectrode, thereby causing tissue located between the return electrodeand the porous electrode to be heated.
 30. A system for ablating bodytissue comprisinga catheter tube having a distal end, a returnelectrode, a fluid source of a medium containing ions, a porouselectrode on the distal end of the catheter tube adapted to beelectrically coupled to the return electrode through tissue, the porouselectrode comprising a wall comprising a distal region and a proximalregion, the wall having an exterior peripherally surrounding an interiorarea, the wall adapted to selectively assume an expanded geometry havinga first maximum diameter and a collapsed geometry having a secondmaximum diameter less than the first maximum diameter, and a lumencommunicating with the interior area and the fluid source to convey intothe interior area the medium containing ions, the wall including aporous section sized to pass ions contained in the medium, the poroussection occupying more of the distal region of the wall than theproximal region, and means for coupling the medium within the interiorarea to a source of electrical energy to establish ionic transport ofelectrical energy from the electrically conductive element through themedium to the exterior of the wall for transmission to the returnelectrode, thereby causing tissue located between the return electrodeand the porous electrode to be ablated.
 31. A system for ablating hearttissue comprisinga catheter tube having a distal end for deployment in aheart chamber, a return electrode, a fluid source of a medium containingions, a porous electrode on the distal end of the catheter tube adaptedto be electrically coupled to the return electrode through heart tissue,the porous electrode comprising a wall comprising a distal region and aproximal region, the wall having an exterior peripherally surrounding aninterior area, the wall adapted to selectively assume an expandedgeometry having a first maximum diameter and a collapsed geometry havinga second maximum diameter less than the first maximum diameter, and alumen communicating with the interior area and the fluid source toconvey into the interior area the medium containing ions, the wallincluding a porous section sized to pass ions contained in the medium,the porous section occupying more of the distal region of the wall thanthe proximal region, and means for coupling the medium within theinterior area to a source of electrical energy to establish ionictransport of electrical energy from the electrically conductive elementthrough the medium to the exterior of the wall for transmission to thereturn electrode, thereby causing heart tissue located between thereturn electrode and the porous electrode to be ablated.
 32. A systemaccording to claim 29 or 30 or 31,wherein at wherein at least 1/3rd ofthe proximal region of the wall is free of pores.
 33. A system accordingto claim 29 or 30 or 31wherein the porous section comprises at leastfirst and second porous zones spaced apart by a third zone free ofpores.
 34. A system according to claim 33wherein the structure includesan axis, and wherein the first and second porous zones arecircumferentially spaced apart by the third zone about the axis.
 35. Asystem according to claim 33wherein the structure includes an axis, andwherein the first and second porous zones are spaced apart by the thirdzone along the axis.
 36. A system according to claim 29 or 30 or31wherein the wall is electrically conductive.
 37. A system according toclaim 29 or 30 or 31and further including a radiopaque element carriedby the structure.
 38. A system according to claim 29 or 30 or 31whereinthe medium carries a radiopaque contrast substance.
 39. A systemaccording to claim 29 or 30 or 31wherein the porous section has anelectrical resistivity of at least about 500 ohm·cm.
 40. A systemaccording to claim 29 or 30 or 31wherein the porous section has anelectrical resistivity of less than about 500 ohm·cm.
 41. A systemaccording to claim 29 or 30 or 31wherein the medium comprises ahypertonic solution.
 42. A system according to claim 41wherein thehypertonic solution includes sodium chloride.
 43. A system according toclaim 42wherein the sodium chloride is present in a concentration at ornear saturation.
 44. A system according to claim 42wherein the sodiumchloride is present in a concentration of up to about 9% weight byvolume.
 45. A system according to claim 29 or 30 or 31wherein thehypertonic solution includes potassium chloride.
 46. A system accordingto claim 29 or 30 or 31wherein the medium has a resistivity lower thanabout 150 ohm·cm.
 47. A system according to claim 46wherein the mediumhas a resistivity lower than about 10 ohm·cm.
 48. A system according toclaim 46wherein the medium has a resistivity lower than about 5 ohm·cm.49. A system according to claim 29 or 30 or 31wherein the mediumincludes a material whose presence increases viscosity of the medium.50. A system according to claim 29 or 30 or 31wherein the mediumincludes at least one ionic material whose presence increases viscosityof the medium.
 51. A system according to claim 50,wherein the at leastone ionic material comprises a radiopaque substance.
 52. A systemaccording to claim 29 or 30 or 31wherein the medium includes a nonionicmaterial whose presence increases viscosity of the medium.
 53. A systemaccording to claim 52wherein the nonionic material includes glycerol.54. A system according to claim 52wherein the nonionic material includesmannitol.
 55. A system according to claim 29 or 30 or 31wherein theporous section comprises an ultraporous material.
 56. A system accordingto claim 29 or 30 or 31wherein the porous section comprises amicroporous material.
 57. A system according to claim 29 or 30 or 31andfurther including a controller including means for specifying ionictransport through the porous section at a desired rate based, at leastin part, upon a desired physiological effect.
 58. A system according toclaim 57wherein the controller includes means for specifying a pressuredifference across the porous section to achieve a desired rate of liquidperfusion based, at least in part, upon the desired physiologicaleffect.
 59. A system according to claim 57wherein the controllerincludes means for specifying the viscosity of the medium independent,at least in part, upon the desired physiological effect.
 60. A systemaccording to claim 29 or 30 or 31and further including a controllerincluding means for specifying an electrical resistivity for the poroussection based, at least in part, upon a desired physiological effect.61. A system according to claim 30 or 31and further including acontroller including means for specifying a first electrical resistivityfor the porous section to achieve a first tissue lesion characteristicand specifying a second electrical resistivity for the porous sectiondifferent than the first electrical resistivity to achieve a secondtissue lesion characteristic different than the first lesioncharacteristic.
 62. A system according to claim 30 or 31and furtherincluding a controller for specifying a first electrical resistivity forthe porous section to achieve a deep tissue lesion geometry andspecifying a second electrical resistivity for the porous sectiongreater than the first electrical resistivity to achieve a shallowtissue lesion geometry.
 63. A system according to claim 29 or 30 or31and further including a temperature sensing element carried by theelectrode, and further including a controller including means forspecifying delivery of electrical energy to the medium based, at leastin part, upon temperature sensed by the temperature sensing element. 64.A system according to claim 29 or 30 or 31wherein the porous section ishydrophilic.
 65. A system according to claim 64wherein the mediumoccupies the interior area subject to interior pressure, wherein theporous section has a bubble point value, and wherein the bubble pointvalue exceeds the interior pressure.
 66. A system according to claim 29or 30 or 31wherein the porous section is hydrophobic.
 67. A systemaccording to claim 66wherein the medium occupies the interior areasubject to interior pressure, wherein the porous section has a bubblepoint value, and wherein the bubble point value is equal to or less thanthe interior pressure.
 68. A system according to claim 29 or 30 or31wherein the medium occupies the interior area subject to interiorpressure, wherein the porous section has a bubble point value, andwherein the bubble point value exceeds the interior pressure.
 69. Asystem according to claim 29 or 30 or 31wherein the medium occupies theinterior area subject to interior pressure, wherein the porous sectionhas a bubble point value, and wherein the bubble point value is equal toor less than the interior pressure.
 70. A system according to claim 29or 30 or 31wherein the porous section includes a hydrophilic coating.71. A system according to claim 29 or 30 or 31wherein the wall ishydrophobic, and wherein the porous section includes a hydrophiliccoating.
 72. A method for heating body tissue comprising the stepsofproviding a catheter tube having a distal end that carries a porouselectrode comprising a wall comprising a distal region and a proximalregion, the wall having an exterior peripherally surrounding an interiorarea, the wall adapted to selectively assume an expanded geometry havinga first maximum diameter and a collapsed geometry having a secondmaximum diameter less than the first maximum diameter, an electricallyconductive element carried within the interior area adapted to transmitelectrical energy, the wall including a porous section sized to passions, the porous section occupying more of the distal region of the wallthan the proximal region, electrically coupling a source of radiofrequency energy to the electrically conductive element to a returnelectrode in contact with body tissue, guiding the catheter tube into abody while causing the porous electrode to assume the collapsedgeometry, causing the porous electrode to assume the expanded geometrywith at least part of the porous section oriented in association withbody tissue, conveying a medium containing ions into the interior area,and ohmically heating body tissue by transmitting radio frequency energyto the electrically conductive element for ionic transport through themedium and porous section to the exterior of the wall for transmissionto the return electrode.
 73. A method for ablating tissue comprising thesteps ofproviding a catheter tube having a distal end that carries aporous electrode comprising a wall comprising a distal region and aproximal region, the wall having an exterior peripherally surrounding aninterior area, the wall adapted to selectively assume an expandedgeometry having a first maximum diameter and a collapsed geometry havinga second maximum diameter less than the first maximum diameter, anelectrically conductive element carried within the interior area adaptedto transmit electrical energy, the wall including a porous section sizedto pass ions, the porous section occupying more of the distal region ofthe wall than the proximal region, electrically coupling a source ofradio frequency energy to the electrically conductive element to areturn electrode in contact with body tissue, guiding the catheter tubeinto a body while causing the porous electrode to assume the collapsedgeometry, causing the porous electrode to assume the expanded geometrywith at least part of the porous section oriented in association withbody tissue, conveying a medium containing ions into the interior area,and ohmically ablating body tissue by transmitting radio frequencyenergy to the electrically conductive element for ionic transportthrough the medium and porous section to the exterior of the wall fortransmission to the return electrode.
 74. A method for ablating hearttissue comprising the steps ofproviding a catheter tube having a distalend that carries a porous electrode comprising a wall comprising adistal region and a proximal region, the wall having an exteriorperipherally surrounding an interior area, the wall adapted toselectively assume an expanded geometry having a first maximum diameterand a collapsed geometry having a second maximum diameter less than thefirst maximum diameter, an electrically conductive element carriedwithin the interior area adapted to transmit electrical energy, the wallincluding a porous section sized to pass ions, the porous sectionoccupying more of the distal region of the wall than the proximalregion, electrically coupling a source of radio frequency energy to theelectrically conductive element to a return electrode in contact withbody tissue, guiding the catheter tube into a heart chamber whilecausing the porous electrode to assume the collapsed geometry, causingthe porous electrode to assume the expanded geometry with at least partof the porous section oriented in association with heart tissue,conveying a medium containing ions into the interior area, and ohmicallyablating heart tissue by transmitting radio frequency energy to theelectrically conductive element for ionic transport through the mediumand porous section to the exterior of the wall for transmission to thereturn electrode.
 75. A method according to claim 73 or 74and furtherincluding the step of specifying a first electrical resistivity for theporous section to achieve a first tissue lesion characteristic andspecifying a second electrical resistivity for the porous sectiondifferent than the first electrical resistivity to achieve a secondtissue lesion characteristic different than the first lesioncharacteristic.
 76. A method according to claim 73 or 74and furtherincluding the step of specifying a first electrical resistivity for theporous section to achieve a deep tissue lesion geometry and specifying asecond electrical resistivity for the porous section greater than thefirst electrical resistivity to achieve a shallow tissue lesiongeometry.
 77. A method according to claim 72 or 73 or 74and furtherincluding the step of specifying ionic transport through the poroussection at a desired rate based, at least in part, upon a desiredphysiological effect.
 78. A system according to claim 72 or 73 or 74 andfurther including the step of specifying an electrical resistivity forthe porous section based, at least in part, upon a desired physiologicaleffect.
 79. A method according to claim 72 or 73 or 74 and furtherincluding the steps ofsensing temperature using a sensing elementcarried by the electrode, and specifying transmission of radio frequencyenergy based, at least in part, upon temperature sensed by the sensingelement.